CMake: fix usage of OPENGL_GLU_INCLUDE_DIR (#1001)

This commit consists of two parts:

1. CMake/resources.cmake: remove the old variable OPENGL_GLU_INCLUDE_DIR
   from the CMake cache if possible, otherwise enforce a clean build.

2. src/CMakeLists.txt, CMake/options.cmake: use the new variable name
   FLTK_OPENGL_GLU_INCLUDE_DIR.

For details please see GitHub Issue #1001.

2 files changed, 58 insertions, 3 deletions

diff --git a/CMake/options.cmake b/CMake/options.cmake
index 2a648baba..c6b6ff60d 100644
--- a/CMake/options.cmake
+++ b/CMake/options.cmake
@@ -611,7 +611,7 @@ endif(FLTK_OPTION_SVG)
 
 #######################################################################
 
-# FIXME: GL libs have already been searched in resources.cmake
+# FIXME: GLU libs have already been searched in resources.cmake
 
 set(HAVE_GL LIB_GL OR LIB_MesaGL)
 set(FLTK_USE_GL FALSE)
@@ -681,8 +681,8 @@ set(FLTK_GL_FOUND FALSE)
 
 if(OPENGL_FOUND)
   set(FLTK_GL_FOUND TRUE)
-  find_path(OPENGL_GLU_INCLUDE_DIR NAMES GL/glu.h OpenGL/glu.h HINTS ${OPENGL_INCLUDE_DIR} ${X11_INCLUDE_DIR})
-  set(CMAKE_REQUIRED_INCLUDES ${OPENGL_INCLUDE_DIR}/GL ${OPENGL_GLU_INCLUDE_DIR})
+  find_path(FLTK_OPENGL_GLU_INCLUDE_DIR NAMES GL/glu.h OpenGL/glu.h HINTS ${OPENGL_INCLUDE_DIR} ${X11_INCLUDE_DIR})
+  set(CMAKE_REQUIRED_INCLUDES ${OPENGL_INCLUDE_DIR}/GL ${FLTK_OPENGL_GLU_INCLUDE_DIR})
 
   if(WIN32)
     list(APPEND GLLIBS -lglu32 -lopengl32)
diff --git a/CMake/resources.cmake b/CMake/resources.cmake
index eaea18c07..c75dfc3f3 100644
--- a/CMake/resources.cmake
+++ b/CMake/resources.cmake
@@ -46,6 +46,61 @@ include(FindPkgConfig)
 # fl_debug_var(PKG_CONFIG_VERSION_STRING)
 
 #######################################################################
+# GitHub Issue #1001: try to "repair" the CMake Cache
+#######################################################################
+#
+# Note: we renamed "our" CMake cache variable OPENGL_GLU_INCLUDE_DIR
+# to FLTK_OPENGL_GLU_INCLUDE_DIR because the former is now defined
+# in find_package(OpenGL) (FindOpenGL.cmake) since CMake 3.29.0.
+#
+# We can remove "our" cache variable if any of these conditions is true:
+#
+#  - CMAKE_VERSION < 3.29
+#  - FLTK_OPENGL_GLU_INCLUDE_DIR is undefined (first run after rename)
+#  - OPENGL_GLU_INCLUDE_DIR is FALSE, i.e. OPENGL_GLU_INCLUDE_DIR-NOTFOUND.
+#
+# Otherwise we can't be sure to remove the *correct* definition of
+# OPENGL_GLU_INCLUDE_DIR and we force the user to rebuild in a clean
+# CMake build directory. This should rarely happen.
+#
+# FIXME: we can remove this code some time after the release of FLTK 1.4.0.
+
+### DEBUG:
+if(DEFINED OPENGL_GLU_INCLUDE_DIR)
+  set(OPENGL_GLU_INCLUDE_DIR_DEFINED TRUE)
+endif()
+
+if(DEFINED FLTK_OPENGL_GLU_INCLUDE_DIR)
+  set(FLTK_OPENGL_GLU_INCLUDE_DIR_DEFINED TRUE)
+endif()
+
+fl_debug_var(OPENGL_FOUND)
+fl_debug_var(OPENGL_INCLUDE_DIR)
+
+fl_debug_var(OPENGL_GLU_INCLUDE_DIR_DEFINED)
+fl_debug_var(OPENGL_GLU_INCLUDE_DIR)
+
+fl_debug_var(FLTK_OPENGL_GLU_INCLUDE_DIR_DEFINED)
+fl_debug_var(FLTK_OPENGL_GLU_INCLUDE_DIR)
+### /DEBUG
+
+if(DEFINED OPENGL_GLU_INCLUDE_DIR)
+
+  if(CMAKE_VERSION VERSION_LESS 3.29 OR
+     (NOT DEFINED FLTK_OPENGL_GLU_INCLUDE_DIR) OR
+     (NOT OPENGL_GLU_INCLUDE_DIR))
+    # message(STATUS "**** Removing OPENGL_GLU_INCLUDE_DIR from CMake cache ****")
+    unset(OPENGL_GLU_INCLUDE_DIR)
+    unset(OPENGL_GLU_INCLUDE_DIR CACHE)
+  else()
+    message(STATUS "")
+    message(NOTICE "FLTK configure: CMake cache inconsistency detected")
+    message(FATAL_ERROR "Please rebuild FLTK in a clean CMake build directory")
+  endif()
+
+endif() # (DEFINED OPENGL_GLU_INCLUDE_DIR)
+
+#######################################################################
 # Find header files...
 #######################################################################