Make the 'Forms' compatibility library 'fltk_forms' optional

This library is no longer needed in FLTK itself. There is only one
demo program (test/forms.cxx) that uses it if it is built. The demo
program displays a message if fltk_forms is not available.

The default is 'enabled' for backwards compatibility but this may
be changed to 'disabled' in a future version.

11 files changed, 152 insertions, 59 deletions

diff --git a/CMake/options.cmake b/CMake/options.cmake
index 8e8ee02e4..0a774b09b 100644
--- a/CMake/options.cmake
+++ b/CMake/options.cmake
@@ -396,13 +396,21 @@ option (OPTION_BUILD_SHARED_LIBS
 )
 
 #######################################################################
-option (OPTION_PRINT_SUPPORT "allow print support" ON)
-option (OPTION_FILESYSTEM_SUPPORT "allow file system support" ON)
 
-option (FLTK_BUILD_FLUID        "Build FLUID"              ON)
-option (FLTK_BUILD_FLTK_OPTIONS "Build fltk-options"       ON)
-option (FLTK_BUILD_TEST         "Build test/demo programs" ON)
-option (FLTK_BUILD_EXAMPLES     "Build example programs"   OFF)
+option (OPTION_PRINT_SUPPORT      "allow print support"               ON)
+option (OPTION_FILESYSTEM_SUPPORT "allow file system support"         ON)
+
+option (FLTK_BUILD_FORMS          "Build forms compatibility library" ON)
+option (FLTK_BUILD_FLUID          "Build FLUID"                       ON)
+option (FLTK_BUILD_FLTK_OPTIONS   "Build fltk-options"                ON)
+option (FLTK_BUILD_TEST           "Build test/demo programs"          ON)
+option (FLTK_BUILD_EXAMPLES       "Build example programs"            OFF)
+
+if (FLTK_BUILD_FORMS)
+  set (FLTK_HAVE_FORMS 1)
+else ()
+  set (FLTK_HAVE_FORMS 0)
+endif ()
 
 if (DEFINED OPTION_BUILD_EXAMPLES)
   message (WARNING
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d2ae79f14..210cfa0d9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -219,34 +219,40 @@ message ("")
 set (_descr "${PROJECT_NAME} ${FLTK_VERSION} generated by CMake ${CMAKE_VERSION}")
 feature_summary (WHAT ALL DESCRIPTION "Configuration Summary for ${_descr} --\n")
 
-message (STATUS "Static libraries will be built in ${CMAKE_CURRENT_BINARY_DIR}/lib")
+message (STATUS "Static libraries  will be built in ${CMAKE_CURRENT_BINARY_DIR}/lib")
 
 if (OPTION_BUILD_SHARED_LIBS)
-  message (STATUS "Shared libraries will be built in ${CMAKE_CURRENT_BINARY_DIR}/lib")
+  message (STATUS "Shared libraries  will be built in ${CMAKE_CURRENT_BINARY_DIR}/lib")
 else ()
-  message (STATUS "Shared libraries will not be built (set OPTION_BUILD_SHARED_LIBS=ON to build)")
+  message (STATUS "Shared libraries  will not be built (set OPTION_BUILD_SHARED_LIBS=ON to build)")
+endif ()
+
+if (FLTK_BUILD_FORMS)
+  message (STATUS "The forms library will be built in ${CMAKE_CURRENT_BINARY_DIR}/lib")
+else ()
+  message (STATUS "The forms library will not be built (set FLTK_BUILD_FORMS=ON to build)")
 endif ()
 
 if (FLTK_BUILD_FLUID)
-  message (STATUS "FLUID            will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/fluid")
+  message (STATUS "FLUID             will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/fluid")
 else ()
-  message (STATUS "FLUID            will not be built (set FLTK_BUILD_FLUID=ON to build)")
+  message (STATUS "FLUID             will not be built (set FLTK_BUILD_FLUID=ON to build)")
 endif ()
 
 if (FLTK_BUILD_FLTK_OPTIONS)
-  message (STATUS "fltk-options     will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/fltk-options")
+  message (STATUS "fltk-options      will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/fltk-options")
 else ()
-  message (STATUS "fltk-options     will not be built (set FLTK_BUILD_FLTK_OPTIONS=ON to build)")
+  message (STATUS "fltk-options      will not be built (set FLTK_BUILD_FLTK_OPTIONS=ON to build)")
 endif ()
 
 if (FLTK_BUILD_TEST)
-  message (STATUS "Test programs    will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/test")
+  message (STATUS "Test programs     will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/test")
 endif ()
 
 if (FLTK_BUILD_EXAMPLES)
-  message (STATUS "Example programs will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/examples")
+  message (STATUS "Example programs  will be built in ${CMAKE_CURRENT_BINARY_DIR}/bin/examples")
 else ()
-  message (STATUS "Example programs will not be built (set FLTK_BUILD_EXAMPLES=ON to build)")
+  message (STATUS "Example programs  will not be built (set FLTK_BUILD_EXAMPLES=ON to build)")
 endif ()
 
 message (STATUS "")
diff --git a/FL/forms.H b/FL/forms.H
index 594ef0c6d..9523b4da5 100644
--- a/FL/forms.H
+++ b/FL/forms.H
@@ -1,7 +1,7 @@
 //
 // Forms emulation header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2011 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -47,7 +47,8 @@ typedef Fl_Window FL_FORM;
 #define FL_CANCEL       0
 #define FL_INVALID      0
 #define FL_IGNORE       -1
-//#define FL_CLOSE      -2 // this variable is never used in FLTK Forms. It is removed becaus it conflicts with the window FL_CLOSE event
+// #define FL_CLOSE     -2 // this variable is never used in FLTK Forms. It is removed
+                           // because it conflicts with the window FL_CLOSE event
 
 #define FL_LCOL         FL_BLACK
 #define FL_COL1         FL_GRAY
diff --git a/configure.ac b/configure.ac
index afeac90d0..fe9b7aeb6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -93,7 +93,7 @@ AS_IF([test x$enable_cp936 = xyes], [
     CFLAGS="$CFLAGS -DCP936"
 ])
 
-AC_ARG_ENABLE([cygwin], AS_HELP_STRING([--enable-cygwin], [use the Cygwin libraries (default=no)]))
+AC_ARG_ENABLE([cygwin], AS_HELP_STRING([--enable-cygwin], [use the Cygwin DLL (default=no)]))
 
 AC_ARG_ENABLE([debug], AS_HELP_STRING([--enable-debug], [turn on debugging]))
 AS_IF([test x$enable_debug = xyes], [
@@ -102,6 +102,8 @@ AS_IF([test x$enable_debug = xyes], [
     DEBUGFLAG=""
 ])
 
+AC_ARG_ENABLE([forms], AS_HELP_STRING([--disable-forms], [build Forms compatibility library (default=yes)]))
+
 AC_ARG_ENABLE([gl], AS_HELP_STRING([--disable-gl], [turn off OpenGL support]))
 
 AC_ARG_ENABLE([localjpeg], AS_HELP_STRING([--enable-localjpeg], [use local JPEG library (default=auto)]))
@@ -145,9 +147,9 @@ AC_ARG_ENABLE([xrender], AS_HELP_STRING([--disable-xrender], [turn off Xrender s
 AC_ARG_ENABLE([fluid], AS_HELP_STRING([--disable-fluid], [turn off fluid building]))
 
 AS_CASE([$host_os], [cygwin* | mingw*], [
-  AC_ARG_ENABLE([gdiplus], AS_HELP_STRING([--disable-gdiplus], [don't use GDI+ for antialiased graphics]))
+    AC_ARG_ENABLE([gdiplus], AS_HELP_STRING([--disable-gdiplus], [don't use GDI+ for antialiased graphics]))
 
- gdiplus_found=no
+    gdiplus_found=no
     AS_IF([test x$enable_gdiplus != xno], [
         AC_CHECK_HEADERS([wtypes.h gdiplus.h], [
             AC_DEFINE([USE_GDIPLUS])
@@ -170,7 +172,6 @@ AS_IF([test "x$with_archflags" != x], [
     CFLAGS="$CFLAGS $with_archflags"
 ])
 
-
 dnl FLTK build options to be used in Makefiles (defined in makeinclude)
 BUILD=""
 
@@ -200,24 +201,37 @@ AS_CASE([$host_os], [cygwin*], [
 
 dnl Define the libraries and link options we will need.
 LINKFLTK="../lib/libfltk.a"
-LINKFLTKFORMS="../lib/libfltk_forms.a"
 LINKFLTKGL="../lib/libfltk_gl.a"
 LINKFLTKIMG="../lib/libfltk_images.a"
 GLDEMOS="gldemos"
 
 LIBEXT=".a"
 LIBNAME="../lib/libfltk.a"
-FLLIBNAME="../lib/libfltk_forms.a"
 GLLIBNAME="../lib/libfltk_gl.a"
 IMGLIBNAME="../lib/libfltk_images.a"
 CAIROLIBNAME="../lib/libfltk_cairo.a"
 
 LIBBASENAME="libfltk.a"
-FLLIBBASENAME="libfltk_forms.a"
 GLLIBBASENAME="libfltk_gl.a"
 IMGLIBBASENAME="libfltk_images.a"
 CAIROLIBBASENAME="libfltk_cairo.a"
 
+dnl set library names etc. for the optional forms library and set 'build_forms'
+dnl to make conditional code independent of the default value of 'enable_forms'
+dnl which is 'yes' (enabled) in 1.4.0 (default may be changed later)
+AS_IF([test x$enable_forms != xno], [
+    build_forms="yes"
+    LINKFLTKFORMS="../lib/libfltk_forms.a"
+    FLLIBNAME="../lib/libfltk_forms.a"
+    FLLIBBASENAME="libfltk_forms.a"
+    AC_DEFINE([FLTK_HAVE_FORMS])
+], [
+    build_forms="no"
+    LINKFLTKFORMS=""
+    FLLIBNAME=""
+    FLLIBBASENAME=""
+])
+
 
 dnl Check for Cairo library unless disabled...
 CAIRODIR=""
@@ -261,7 +275,6 @@ AC_SUBST(CAIROLIBS)
 AC_SUBST(LINKFLTKCAIRO)
 AC_SUBST(FLTKCAIROOPTION)
 
-AC_SUBST(FLLIBNAME)
 AC_SUBST(GLDEMOS)
 AC_SUBST(GLLIBNAME)
 AC_SUBST(IMGLIBNAME)
@@ -269,12 +282,10 @@ AC_SUBST(CAIROLIBNAME)
 AC_SUBST(LIBEXT)
 AC_SUBST(LIBNAME)
 AC_SUBST(LINKFLTK)
-AC_SUBST(LINKFLTKFORMS)
 AC_SUBST(LINKFLTKGL)
 AC_SUBST(LINKFLTKIMG)
 
 AC_SUBST(LIBBASENAME)
-AC_SUBST(FLLIBBASENAME)
 AC_SUBST(GLLIBBASENAME)
 AC_SUBST(IMGLIBBASENAME)
 AC_SUBST(CAIROLIBBASENAME)
@@ -432,7 +443,7 @@ AS_IF([test "x$enable_shared" = xyes], [
         DSOCOMMAND="\$(CXX) \$(DSOFLAGS) -Wl,-soname,\$@ \$(LDLIBS) -shared $DEBUGFLAG -o"
     ])
 
-    LINKSHARED="-L../src $FLTKCAIROOPTION -lfltk_images$SHAREDSUFFIX -lfltk_forms$SHAREDSUFFIX -lfltk$SHAREDSUFFIX"
+    LINKSHARED="-L../src $FLTKCAIROOPTION -lfltk_images$SHAREDSUFFIX -lfltk$SHAREDSUFFIX"
 ], [
     DSOCOMMAND="echo"
     DSOLINK=""
@@ -445,9 +456,21 @@ AS_IF([test "x$enable_shared" = xyes], [
     SHAREDSUFFIX=""
     FLUID="fluid"
     FLTK_OPTIONS="fltk-options"
-    LINKSHARED="$LINKFLTKCAIRO ../lib/libfltk_images.a ../lib/libfltk_forms.a ../lib/libfltk.a"
+    LINKSHARED="$LINKFLTKCAIRO ../lib/libfltk_images.a ../lib/libfltk.a"
 ])
 
+dnl reset FLDSONAME if the Forms compatibility library is disabled (not built):
+dnl overwrite the variable because this is easier than adding conditional code above
+AS_IF([test x$build_forms = xno], [
+    FLDSONAME=""
+])
+
+AC_SUBST([FLLIBNAME])
+AC_SUBST([FLLIBBASENAME])
+AC_SUBST([FLDSONAME])
+AC_SUBST([LINKFLTKFORMS])
+AC_SUBST([LINKSHARED])
+
 
 dnl Define the fluid executable used when building the test programs.
 dnl In a native build we use the fluid executable created during the build,
@@ -463,12 +486,10 @@ AC_SUBST([DSOCOMMAND])
 AC_SUBST([DSOFLAGS])
 AC_SUBST([DSOLINK])
 AC_SUBST([DSONAME])
-AC_SUBST([FLDSONAME])
 AC_SUBST([GLDSONAME])
 AC_SUBST([IMGDSONAME])
 AC_SUBST([CAIRODSONAME])
 AC_SUBST([SHAREDSUFFIX])
-AC_SUBST([LINKSHARED])
 AC_SUBST([FLUID])
 AC_SUBST([FLUID_BUILD])
 AC_SUBST([FLTK_OPTIONS])
@@ -1791,12 +1812,13 @@ AS_IF([test x$enable_fluid != xno], [
     echo "    Build fluid: NO"
     ])
 
-dnl
-echo "Cross-compiling: $fltk_cross_compiling"
 AS_IF([test "$fltk_cross_compiling" = "yes"], [
+    echo "Cross-compiling: YES"
     echo "          Build: $build -> Host: $host"
+], [
+    echo "Cross-compiling: NO"
 ])
-dnl
+
 echo "       Graphics: $graphics"
 dnl
 AS_IF([test x$JPEG = x], [
@@ -1814,6 +1836,13 @@ AS_IF([test x$ZLIB = x], [
 ], [
     echo "                 ZLIB=Builtin"
 ])
+
+AS_IF([test x$build_forms = xyes], [
+    echo "Forms library:   YES"
+], [
+    echo "Forms library:   NO"
+])
+
 AS_IF([test x$enable_cairo = xyes], [
     echo "                 CAIRO=lib"
 ])
diff --git a/fl_config.cmake.in b/fl_config.cmake.in
index a6e3b086e..3fdcea274 100644
--- a/fl_config.cmake.in
+++ b/fl_config.cmake.in
@@ -2,7 +2,7 @@
 /*
  * Build configuration file for the Fast Light Tool Kit (FLTK).
  *
- * Copyright 1998-2021 by Bill Spitzak and others.
+ * Copyright 1998-2023 by Bill Spitzak and others.
  *
  * This library is free software. Distribution and use rights are outlined in
  * the file "COPYING" which should have been included with this file.  If this
@@ -47,6 +47,15 @@
 
 
 /*
+ * FLTK_HAVE_FORMS
+ *
+ * Do we have the Forms compatibility library available?
+*/
+
+#cmakedefine FLTK_HAVE_FORMS 1
+
+
+/*
  * FLTK_USE_X11
  *
  * Do we use X11 for the current platform?
diff --git a/fl_config.in b/fl_config.in
index 444d85212..03c3abf57 100644
--- a/fl_config.in
+++ b/fl_config.in
@@ -1,7 +1,7 @@
 /*
  * Build configuration file for the Fast Light Tool Kit (FLTK).
  *
- * Copyright 1998-2021 by Bill Spitzak and others.
+ * Copyright 1998-2023 by Bill Spitzak and others.
  *
  * This library is free software. Distribution and use rights are outlined in
  * the file "COPYING" which should have been included with this file.  If this
@@ -46,6 +46,15 @@
 
 
 /*
+ * FLTK_HAVE_FORMS
+ *
+ * Do we have the Forms compatibility library available?
+*/
+
+#undef FLTK_HAVE_FORMS
+
+
+/*
  * FLTK_USE_X11
  *
  * Do we use X11 for the current platform?
diff --git a/fltk-options/Makefile b/fltk-options/Makefile
index 0c7c9c6da..06b3b710b 100644
--- a/fltk-options/Makefile
+++ b/fltk-options/Makefile
@@ -25,7 +25,7 @@ OBJECTS = $(CPPFILES:.cxx=.o)
 
 all:	$(FLTK_OPTIONS) fltk-options$(EXEEXT)
 
-fltk-options$(EXEEXT):	$(OBJECTS) $(LIBNAME) $(FLLIBNAME)
+fltk-options$(EXEEXT):	$(OBJECTS) $(LIBNAME)
 	echo Linking $@...
 	$(CXX) $(ARCHFLAGS) $(CXXFLAGS) -o $@ $(OBJECTS) $(LINKFLTKIMG) $(LDFLAGS) $(LDLIBS)
 	$(OSX_ONLY) $(RM) -r -f fltk-options.app
@@ -34,7 +34,7 @@ fltk-options$(EXEEXT):	$(OBJECTS) $(LIBNAME) $(FLLIBNAME)
 	$(OSX_ONLY) $(INSTALL) icons/fltk-options.icns fltk-options.app/Contents/Resources
 	$(OSX_ONLY) $(INSTALL) fltk-options.plist fltk-options.app/Contents/Info.plist
 
-fltk-options-shared$(EXEEXT):	$(OBJECTS) ../src/$(DSONAME) ../src/$(FLDSONAME)
+fltk-options-shared$(EXEEXT):	$(OBJECTS) ../src/$(DSONAME)
 	echo Linking $@...
 	$(CXX) $(ARCHFLAGS) $(CXXFLAGS) -o $@ $(OBJECTS) $(LINKSHARED) $(LDFLAGS) $(LDLIBS)
 
diff --git a/fluid/Makefile b/fluid/Makefile
index 0fa834ebf..75a785e71 100644
--- a/fluid/Makefile
+++ b/fluid/Makefile
@@ -66,8 +66,7 @@ OBJECTS = $(CPPFILES:.cxx=.o)
 
 all:	$(FLUID) fluid$(EXEEXT)
 
-fluid$(EXEEXT):		$(OBJECTS) $(LIBNAME) $(FLLIBNAME) \
-			$(IMGLIBNAME)
+fluid$(EXEEXT):		$(OBJECTS) $(LIBNAME) $(IMGLIBNAME)
 	echo Linking $@...
 	$(CXX) $(ARCHFLAGS) $(CXXFLAGS) -o $@ $(OBJECTS) $(LINKFLTKIMG) $(LDFLAGS) $(LDLIBS)
 	$(OSX_ONLY) $(RM) -r -f fluid.app
@@ -76,8 +75,7 @@ fluid$(EXEEXT):		$(OBJECTS) $(LIBNAME) $(FLLIBNAME) \
 	$(OSX_ONLY) $(INSTALL) icons/fluid.icns fluid.app/Contents/Resources
 	$(OSX_ONLY) $(INSTALL) fluid.plist fluid.app/Contents/Info.plist
 
-fluid-shared$(EXEEXT):	$(OBJECTS) ../src/$(DSONAME) ../src/$(FLDSONAME) \
-			../src/$(IMGDSONAME)
+fluid-shared$(EXEEXT):	$(OBJECTS) ../src/$(DSONAME) ../src/$(IMGDSONAME)
 	echo Linking $@...
 	$(CXX) $(ARCHFLAGS) $(CXXFLAGS) -o $@ $(OBJECTS) $(LINKSHARED) $(LDFLAGS) $(LDLIBS)
 
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index be2634d62..47d576ac3 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -406,14 +406,18 @@ set (CPPFILES
   ${DRIVER_FILES}
 )
 
-set (FORMS_FILES
-  forms_compatibility.cxx
-  forms_bitmap.cxx
-  forms_free.cxx
-  forms_fselect.cxx
-  forms_pixmap.cxx
-  forms_timer.cxx
-)
+if (FLTK_BUILD_FORMS)
+  set (FORMS_FILES
+    forms_compatibility.cxx
+    forms_bitmap.cxx
+    forms_free.cxx
+    forms_fselect.cxx
+    forms_pixmap.cxx
+    forms_timer.cxx
+  )
+else ()
+  set (FORMS_FILES "")
+endif ()
 
 set (GLCPPFILES
   Fl_Gl_Choice.cxx
@@ -759,8 +763,10 @@ endif()
 
 #######################################################################
 
-FL_ADD_LIBRARY (fltk_forms STATIC "${FORMS_FILES}")
-target_link_libraries (fltk_forms fltk)
+if (FLTK_BUILD_FORMS)
+  FL_ADD_LIBRARY (fltk_forms STATIC "${FORMS_FILES}")
+  target_link_libraries (fltk_forms fltk)
+endif ()
 
 #######################################################################
 
@@ -806,8 +812,10 @@ if (OPTION_BUILD_SHARED_LIBS AND NOT MSVC)
 
   ###################################################################
 
-  FL_ADD_LIBRARY (fltk_forms SHARED "${FORMS_FILES}")
-  target_link_libraries (fltk_forms_SHARED fltk_SHARED)
+  if (FLTK_BUILD_FORMS)
+    FL_ADD_LIBRARY (fltk_forms SHARED "${FORMS_FILES}")
+    target_link_libraries (fltk_forms_SHARED fltk_SHARED)
+  endif ()
 
   ###################################################################
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index be40319ce..7fea98adc 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -75,6 +75,14 @@ else ()
   set (GLDEMO_SHARED fltk_SHARED)
   endif ()
 
+# Similar approach for the 'forms' demo (shared lib not supported)
+
+if (FLTK_BUILD_FORMS)
+  set (FORMS_LIBS fltk_forms) # implies 'fltk' as well
+else ()
+  set (FORMS_LIBS fltk)
+endif ()
+
 #######################################################################
 
 CREATE_EXAMPLE (adjuster adjuster.cxx fltk)
@@ -106,7 +114,7 @@ CREATE_EXAMPLE (flex_demo flex_demo.cxx fltk)
 CREATE_EXAMPLE (flex_login flex_login.cxx fltk)
 CREATE_EXAMPLE (fltk-versions fltk-versions.cxx fltk)
 CREATE_EXAMPLE (fonts fonts.cxx fltk)
-CREATE_EXAMPLE (forms forms.cxx "fltk_forms;fltk")
+CREATE_EXAMPLE (forms forms.cxx "${FORMS_LIBS}")
 CREATE_EXAMPLE (fullscreen fullscreen.cxx "${GLDEMO_LIBS}")
 CREATE_EXAMPLE (grid_alignment grid_alignment.cxx fltk)
 CREATE_EXAMPLE (grid_buttons grid_buttons.cxx fltk)
diff --git a/test/forms.cxx b/test/forms.cxx
index 84f623649..e19fade77 100644
--- a/test/forms.cxx
+++ b/test/forms.cxx
@@ -19,10 +19,13 @@
 //     https://www.fltk.org/bugs.php
 //
 
-#include <FL/forms.H> // changed for FLTK
+#include <FL/Fl.H>      // FLTK: includes <FL/fl_config.h>
+#ifdef FLTK_HAVE_FORMS  // FLTK: defined in <FL/fl_config.h> since FLTK 1.4.0
 
-static int border = 1; // changed from FL_TRANSIENT for FLTK
-// (this is so the close box and Esc work to close the window)
+#include <FL/forms.H>   // changed for FLTK
+
+static int border = 1;  // changed from FL_TRANSIENT for FLTK
+                        // (this is so the close box and Esc work to close the window)
 
 typedef struct { Fl_Boxtype val; const char *name; } VN_struct;
 
@@ -173,4 +176,18 @@ int main(int argc, char *argv[]) {
     ;
 
   return 0;
+
+}
+
+#else // (!FLTK_HAVE_FORMS)
+
+#include <FL/fl_ask.H>
+
+int main(int argc, char **argv) {
+  fl_message_title("This program needs the Forms compatibility library");
+  fl_message(
+    "Please configure FLTK with Forms enabled (--enable-forms)\n"
+    "or the CMake option FLTK_BUILD_FORMS=ON.");
+  return 0;
 }
+#endif // (FLTK_HAVE_FORMS)