FindOpenGL: Default to non-GLVND libraries for legacy GL

Projects using `OPENGL_LIBRARIES` or `OpenGL::GL` expect legacy GL.
Although GLVND OpenGL+GLX provides legacy GL interfaces, using those
library files may conflict with legacy GL library files used by
dependencies (or dependents) of such projects.  Therefore we should
not yet use OpenGL+GLX when a legacy GL library is available.

If `OPENGL_gl_LIBRARY` is set then use it as the legacy GL library.
If it is *not* set then fall back to using GLVND OpenGL+GLX to provide
legacy GL interfaces.  This will allow users to build projects using
GLVND even if they have not been ported.

Fixes: #17437

Modules/FindOpenGL.cmake

@@ -53,7 +53,7 @@
 #  Path to the EGL include directory.
 # ``OPENGL_LIBRARIES``
 #  Paths to the OpenGL library, windowing system libraries, and GLU libraries.
-#  On Linux, this assumes glX and is never correct for EGL-based targets.
+#  On Linux, this assumes GLX and is never correct for EGL-based targets.
 #  Clients are encouraged to use the ``OpenGL::*`` import targets instead.
 #
 # Cache variables
@@ -80,12 +80,14 @@
 # context libraries from OpenGL itself; OpenGL lives in "libOpenGL", and
 # contexts are defined in "libGLX" or "libEGL".  GLVND is currently the only way
 # to get OpenGL 3+ functionality via EGL in a manner portable across vendors.
+# Projects may use GLVND explicitly with target ``OpenGL::OpenGL`` and either
+# ``OpenGL::GLX`` or ``OpenGL::EGL``.
 #
-# On Linux systems FindOpenGL defaults to using GLVND if available.  Users can
-# utilize GLVND explicitly with targets ``OpenGL::OpenGL``, ``OpenGL::GLX``, and
-# ``OpenGL::EGL``.  Additionally, when GLVND is available the ``OpenGL::GL``
-# target is equivalent to ``OpenGL::OpenGL OpenGL::GLX``.  When the system is
-# not GLVND-based, ``OpenGL::GL`` expands to libGL as it has historically done.
+# Projects may use the ``OpenGL::GL`` target (or ``OPENGL_LIBRARIES`` variable)
+# to use legacy GL.  By default, these will use the legacy libGL library file.
+# If ``OPENGL_gl_LIBRARY`` is empty or not found and GLVND is available, the
+# ``OpenGL::GL`` target will use GLVND ``OpenGL::OpenGL`` and ``OpenGL::GLX``
+# (and the ``OPENGL_LIBRARIES`` variable will use the corresponding libraries).
 # Thus, for non-EGL-based Linux targets, the ``OpenGL::GL`` target is most
 # portable.
 #
@@ -360,18 +362,8 @@ if(OPENGL_FOUND)
                           "${OPENGL_GLX_INCLUDE_DIR}")
   endif()
 
-  if(TARGET OpenGL::OpenGL AND TARGET OpenGL::GLX AND NOT TARGET OpenGL::GL)
-    # if GLVND with GLX is available, make ::GL a synonym for 'OpenGL::OpenGL
-    # OpenGL::GLX'.
-    add_library(OpenGL::GL INTERFACE IMPORTED)
-    set_target_properties(OpenGL::GL PROPERTIES INTERFACE_LINK_LIBRARIES
-                          OpenGL::OpenGL)
-    set_property(TARGET OpenGL::GL APPEND PROPERTY INTERFACE_LINK_LIBRARIES
-                 OpenGL::GLX)
-    set_target_properties(OpenGL::GL PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
-                          "${OPENGL_INCLUDE_DIR}")
-
-  elseif(NOT TARGET OpenGL::GL)
+  if(OPENGL_gl_LIBRARY AND NOT TARGET OpenGL::GL)
+    # A legacy GL library is available, so use it for the legacy GL target.
     if(IS_ABSOLUTE "${OPENGL_gl_LIBRARY}")
       add_library(OpenGL::GL UNKNOWN IMPORTED)
       if(OPENGL_gl_LIBRARY MATCHES "/([^/]+)\\.framework$")
@@ -392,6 +384,16 @@ if(OPENGL_FOUND)
     endif()
     set_target_properties(OpenGL::GL PROPERTIES
       INTERFACE_INCLUDE_DIRECTORIES "${OPENGL_INCLUDE_DIR}")
+  elseif(NOT TARGET OpenGL::GL AND TARGET OpenGL::OpenGL AND TARGET OpenGL::GLX)
+    # A legacy GL library is not available, but we can provide the legacy GL
+    # target using GLVND OpenGL+GLX.
+    add_library(OpenGL::GL INTERFACE IMPORTED)
+    set_target_properties(OpenGL::GL PROPERTIES INTERFACE_LINK_LIBRARIES
+                          OpenGL::OpenGL)
+    set_property(TARGET OpenGL::GL APPEND PROPERTY INTERFACE_LINK_LIBRARIES
+                 OpenGL::GLX)
+    set_target_properties(OpenGL::GL PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                          "${OPENGL_INCLUDE_DIR}")
   endif()
 
   # ::EGL is a GLVND library, and thus Linux-only: we don't bother checking
@@ -440,10 +442,12 @@ if(OPENGL_FOUND)
   endif()
 
   # OPENGL_LIBRARIES mirrors OpenGL::GL's logic ...
-  if(TARGET OpenGL::GLX AND TARGET OpenGL::OpenGL)
+  if(OPENGL_gl_LIBRARY)
+    set(OPENGL_LIBRARIES ${OPENGL_gl_LIBRARY})
+  elseif(TARGET OpenGL::OpenGL AND TARGET OpenGL::GLX)
     set(OPENGL_LIBRARIES ${OPENGL_opengl_LIBRARY} ${OPENGL_glx_LIBRARY})
   else()
-    set(OPENGL_LIBRARIES ${OPENGL_gl_LIBRARY})
+    set(OPENGL_LIBRARIES "")
   endif()
   # ... and also includes GLU, if available.
   if(TARGET OpenGL::GLU)