CMake: improve linking OpenGL3 example programs (#959)

- Remove find_library(LIB_GLEW ...) from 'resources.cmake'

- Use the variable FLTK_GLEW_DIR to "find" a user supplied GLEW library
  for OpenGL3 example programs (examples/OpenGL3*.cxx).
  Both static and shared libs (DLLs) are supported, but the static
  library is preferred to avoid DLL dependencies.

- Define preprocessor macro 'GLEW_STATIC' by CMake compile definitions
  if and only if the static glew library (glew32s) was found and linked.
  The previous code defined 'GLEW_STATIC' unconditionally in the source
  code which led to undefined references if a shared lib (DLL) was used.

CMake/resources.cmake

@@ -152,14 +152,13 @@ endif((NOT APPLE) OR FLTK_BACKEND_X11)
 find_library(LIB_freetype freetype)
 find_library(LIB_GL GL)
 find_library(LIB_MesaGL MesaGL)
-find_library(LIB_GLEW NAMES GLEW glew32)
 find_library(LIB_jpeg jpeg)
 find_library(LIB_png png)
 find_library(LIB_zlib z)
 find_library(LIB_m m)
 
 mark_as_advanced(LIB_dl LIB_fontconfig LIB_freetype)
-mark_as_advanced(LIB_GL LIB_MesaGL LIB_GLEW)
+mark_as_advanced(LIB_GL LIB_MesaGL)
 mark_as_advanced(LIB_jpeg LIB_png LIB_zlib)
 mark_as_advanced(LIB_m)
 

examples/CMakeLists.txt

@@ -80,7 +80,7 @@ set(FLUID_SOURCES
 )
 
 ############################################################
-# examples requiring fltk_images
+# examples requiring fltk::images
 ############################################################
 
 set(IMAGE_SOURCES
@@ -100,7 +100,7 @@ set(CAIRO_SOURCES
 )
 
 ############################################################
-# examples requiring OpenGL + libGLEW
+# examples requiring OpenGL3 + GLEW
 ############################################################
 
 set(OPENGL_SOURCES
@@ -141,33 +141,79 @@ foreach(src ${CAIRO_SOURCES})
 endforeach(src)
 
 ############################################################
-# create example programs with OpenGL + libGLEW
+# create example programs with OpenGL3 + GLEW
 ############################################################
+#
+# Note 1: macOS (Quartz) does not need GLEW, it's included in OpenGL
+# Note 2: find_package(GLEW) finds either shared or static libs or both.
+# Note 3: on Windows we set the variable GLEW_USE_STATIC_LIBS=TRUE because
+#         we *want* to find static libs but we *can* also use shared libs.
+# Note 4: FindGLEW.cmake changed the library suffixes for MinGW in CMake 3.28.0,
+#         obviously "assuming" that ".lib" is Visual Studio only. There have been
+#         discussions about finding the "wrong" libraries since CMake 3.25 or so.
+#         Therefore the static lib "glew32s.lib" is not found if CMake 3.28 or
+#         higher is used (current version, as of this writing: 3.29.3). However,
+#         this assumption is probably not true for a pure C library (glew32s).
+#         This library *is* found and works well with CMake 3.15.0 - 3.27.9.
+#         Users may need to copy or rename "glew32s.lib" to "glew32s.a"
+#         if CMake 3.28 or higher is used.
+#         Albrecht-S, May 13, 2024
 
-# Note: macOS does not need libGLEW
-
-if(APPLE AND NOT FLTK_BACKEND_X11)
-  if(NOT LIB_GLEW)
-    set(LIB_GLEW TRUE)
+if(OPENGL_FOUND)
+  if(WIN32)
+    set(GLEW_USE_STATIC_LIBS TRUE)
   endif()
-  set(REQUIRED_LIBS fltk::gl)
+  set(_glew_lib GLEW::glew)
+  set(_glew_static FALSE)
+  if(APPLE AND NOT FLTK_BACKEND_X11) # macOS Quartz
+    set(_glew_lib)
+    set(GLEW_FOUND TRUE)
   else()
-  set(REQUIRED_LIBS fltk::gl ${LIB_GLEW})
+    # set(GLEW_VERBOSE TRUE) # make `find_package(GLEW)` verbose
+    set(_CMAKE_PREFIX_PATH_SAVED ${CMAKE_PREFIX_PATH})
+    set(CMAKE_PREFIX_PATH ${FLTK_GLEW_DIR} ${CMAKE_PREFIX_PATH})
+    find_package(GLEW MODULE)
+    set(CMAKE_PREFIX_PATH ${_CMAKE_PREFIX_PATH_SAVED})
+    unset(_CMAKE_PREFIX_PATH_SAVED)
+    # Did we find the static lib? If yes, use it
+    if(TARGET GLEW::glew_s)
+      set(_glew_lib GLEW::glew_s)
+      set(_glew_static TRUE)
+    endif()
   endif()
 
-if(OPENGL_FOUND AND LIB_GLEW)
+  if(0) # Debug
+    fl_debug_var(OPENGL_FOUND)
+    fl_debug_var(GLEW_FOUND)
+    fl_debug_var(GLEW_DIR)
+    fl_debug_var(_glew_lib)
+    fl_debug_target(GLEW::glew)
+    fl_debug_target(GLEW::glew_s)
+    fl_debug_target(GLEW::GLEW)
+  endif() # /Debug
+
+endif(OPENGL_FOUND)
+
+if(OPENGL_FOUND AND (TARGET "${_glew_lib}" OR APPLE))
+
+  # GLEW was found, create the OpenGL3 targets:
+
   foreach(tgt ${OPENGL_SOURCES})
-    fl_create_example(${tgt} ${tgt}.cxx "${REQUIRED_LIBS}")
+    fl_create_example(${tgt} ${tgt}.cxx "fltk::gl;${_glew_lib}")
     set_property(TARGET ${tgt} PROPERTY CXX_STANDARD 11)
     set_property(TARGET ${tgt} PROPERTY CXX_STANDARD_REQUIRED TRUE)
+    # define preprocessor macro GLEW_STATIC only if we link to the static lib
+    if(_glew_static)
+      target_compile_definitions(${tgt} PRIVATE "GLEW_STATIC")
+    endif()
   endforeach(tgt)
 
 else()
-  message(STATUS
-    "OpenGL or libGLEW not present: OpenGL example programs will not be built.")
-  fl_debug_var(OPENGL_FOUND)
-  fl_debug_var(LIB_GLEW)
-  message("")
-endif(OPENGL_FOUND AND LIB_GLEW)
 
-unset(REQUIRED_LIBS)
+  message(STATUS
+    "OpenGL or GLEW not present: OpenGL3 example programs will not be built.")
+  fl_debug_var(OPENGL_FOUND)
+  fl_debug_var(GLEW_FOUND)
+  message("")
+
+endif() # (OPENGL_FOUND AND TARGET "${_glew_lib}")

examples/OpenGL3-glut-test.cxx

@@ -19,11 +19,10 @@
 #  define GL_DO_NOT_WARN_IF_MULTI_GL_VERSION_HEADERS_INCLUDED 1
 #  include <OpenGL/gl3.h> // defines OpenGL 3.0+ functions
 #else
-#  if defined(_WIN32)
-#    define GLEW_STATIC 1
-#  endif
+// Note: GLEW_STATIC is defined by CMake if the static lib is linked
 #  include <GL/glew.h>
 #endif
+
 #include <FL/glut.H>
 #include <FL/fl_ask.H>
 #include <stdio.h>

examples/OpenGL3test.cxx

@@ -25,9 +25,7 @@
 #if defined(__APPLE__)
 #  include <OpenGL/gl3.h> // defines OpenGL 3.0+ functions
 #else
-#  if defined(_WIN32)
-#    define GLEW_STATIC 1
-#  endif
+// Note: GLEW_STATIC is defined by CMake if the static lib is linked
 #  include <GL/glew.h>
 #endif
 #include <FL/gl.h> // for gl_texture_reset()