Introduce "Modern CMake" in FLTK

This is a big commit and there are too many changes to list them all.
The main changes are:

- rename all CMake build options to 'FLTK_*'
- export library targets with namespace (prefix) 'fltk::'
- standardize shared library target names with suffix '-shared'
- set public build properties on libraries for consumers
- document library names and aliases in README.CMake.txt
- document changes in "Migrating Code from FLTK 1.3 to 1.4"
- partial backwards compatibility for old user projects

Included but not directly related changes:

- fix Windows (Visual Studio) DLL build
- add CMake function fl_debug_target() to show target properties
- don't build test programs if FLTK is a subproject
- internal: reformat CMake code: remove space before '('

Thanks to Matthias and Manolo for their help, testing, and feeback.

1 files changed, 60 insertions, 31 deletions

diff --git a/jpeg/CMakeLists.txt b/jpeg/CMakeLists.txt
index f65f71636..c1ec4fae8 100644
--- a/jpeg/CMakeLists.txt
+++ b/jpeg/CMakeLists.txt
@@ -1,7 +1,7 @@
 #
 # JPEG library CMake configuration for the Fast Light Toolkit (FLTK).
 #
-# Copyright 1998-2023 by Bill Spitzak and others.
+# Copyright 1998-2024 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
@@ -18,70 +18,99 @@
 # (For now, let's use the mode that requires the image fit into memory.
 # This is the recommended mode for Win32 anyway.)
 
-set (systemdependent_SRCS jmemnobs.c)
+set(systemdependent_SRCS jmemnobs.c)
 
 # library object files common to compression and decompression
 
-set (common_SRCS
-     jaricom.c jcomapi.c jutils.c jerror.c jmemmgr.c
+set(common_SRCS
+    jaricom.c jcomapi.c jutils.c jerror.c jmemmgr.c
 )
 
 # compression library object files
 
-set (compression_SRCS
-     jcapimin.c jcapistd.c jcarith.c jctrans.c jcparam.c jdatadst.c jcinit.c
-     jcmaster.c jcmarker.c jcmainct.c jcprepct.c jccoefct.c jccolor.c
-     jcsample.c jchuff.c jcdctmgr.c jfdctfst.c jfdctflt.c
-     jfdctint.c
+set(compression_SRCS
+    jcapimin.c jcapistd.c jcarith.c jctrans.c jcparam.c jdatadst.c jcinit.c
+    jcmaster.c jcmarker.c jcmainct.c jcprepct.c jccoefct.c jccolor.c
+    jcsample.c jchuff.c jcdctmgr.c jfdctfst.c jfdctflt.c
+    jfdctint.c
 )
 
 # decompression library object files
 
-set (decompression_SRCS
-     jdapimin.c jdapistd.c jdarith.c jdtrans.c jdatasrc.c jdmaster.c
-     jdinput.c jdmarker.c jdhuff.c jdmainct.c jdcoefct.c
-     jdpostct.c jddctmgr.c jidctfst.c jidctflt.c jidctint.c
-     jdsample.c jdcolor.c jquant1.c jquant2.c jdmerge.c
+set(decompression_SRCS
+    jdapimin.c jdapistd.c jdarith.c jdtrans.c jdatasrc.c jdmaster.c
+    jdinput.c jdmarker.c jdhuff.c jdmainct.c jdcoefct.c
+    jdpostct.c jddctmgr.c jidctfst.c jidctflt.c jidctint.c
+    jdsample.c jdcolor.c jquant1.c jquant2.c jdmerge.c
 )
 
-list (APPEND BUILD_SRCS "${systemdependent_SRCS};${common_SRCS}")
-list (APPEND BUILD_SRCS "${compression_SRCS};${decompression_SRCS}")
+list(APPEND BUILD_SRCS "${systemdependent_SRCS};${common_SRCS}")
+list(APPEND BUILD_SRCS "${compression_SRCS};${decompression_SRCS}")
 
 #######################################################################
+# Set common variables for static and shared builds
+#######################################################################
+
+set(_include_dirs
+    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+    $<INSTALL_INTERFACE:include>
+)
 
-# Suppress some Visual Studio compiler warnings
+# Suppress Visual Studio compiler warnings
 
-set (msvc_warnings /wd4267)
+set(msvc_warnings /wd4267)
 
 #######################################################################
 # Build the static library
 #######################################################################
 
-FL_ADD_LIBRARY (fltk_jpeg STATIC "${BUILD_SRCS}")
+fl_add_library(fltk_jpeg STATIC "${BUILD_SRCS}")
+set(target     fltk_jpeg)
+
+target_include_directories(${target} PUBLIC ${_include_dirs})
+
+if(MSVC)
+  target_compile_options(${target} PRIVATE ${msvc_warnings})
+endif(MSVC)
 
-if (MSVC)
-  target_compile_options (fltk_jpeg PRIVATE ${msvc_warnings})
-endif (MSVC)
+list(APPEND FLTK_IMAGE_LIBRARIES fltk::jpeg)
+
+# Propagate to parent scope (modified by fl_add_library and here)
+set(FLTK_LIBRARIES       ${FLTK_LIBRARIES}       PARENT_SCOPE)
+set(FLTK_IMAGE_LIBRARIES ${FLTK_IMAGE_LIBRARIES} PARENT_SCOPE)
 
 #######################################################################
 # Build the shared library (optional)
 #######################################################################
 
-if (OPTION_BUILD_SHARED_LIBS)
+if(FLTK_BUILD_SHARED_LIBS)
+
+  # ensure to export all symbols for Windows DLL's
+  set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
+
+  fl_add_library(fltk_jpeg SHARED "${BUILD_SRCS}")
+  set(target     fltk_jpeg-shared)
+
+  target_include_directories(${target} PUBLIC ${_include_dirs})
+
+  if(MSVC)
+    target_compile_options(${target} PRIVATE ${msvc_warnings})
+  endif(MSVC)
 
-  FL_ADD_LIBRARY (fltk_jpeg SHARED "${BUILD_SRCS}")
+  list(APPEND FLTK_IMAGE_LIBRARIES_SHARED fltk::jpeg-shared)
 
-  if (MSVC)
-    target_compile_options (fltk_jpeg_SHARED PRIVATE ${msvc_warnings})
-  endif (MSVC)
+  # Propagate to parent scope (modified by fl_add_library and here)
+  set(FLTK_LIBRARIES_SHARED       ${FLTK_LIBRARIES_SHARED}       PARENT_SCOPE)
+  set(FLTK_IMAGE_LIBRARIES_SHARED ${FLTK_IMAGE_LIBRARIES_SHARED} PARENT_SCOPE)
 
-endif (OPTION_BUILD_SHARED_LIBS)
+endif(FLTK_BUILD_SHARED_LIBS)
 
 #######################################################################
 # Install the library headers
 #######################################################################
 
-install (FILES jconfig.h jerror.h jmorecfg.h jpeglib.h
-               fltk_jpeg_prefix.h
-         DESTINATION ${FLTK_INCLUDEDIR}/FL/images
+install(FILES
+          jconfig.h jerror.h jmorecfg.h jpeglib.h fltk_jpeg_prefix.h
+        DESTINATION
+          ${FLTK_INCLUDEDIR}/FL/images
 )