FindOpenGL: Add support for a separate GLU include directory

Add a OPENGL_GLU_INCLUDE_DIR cache variable for `GL/glu.h` and attach it
as an include directory to the `OpenGL::GLU` target.  Also add a
`OPENGL_INCLUDE_DIRS` result variable to list all include directories.
Account for the GLU header when deciding whether GLU is found.

Fixes: #25627

Help/release/dev/findopengl-glu-include-dir.rst

@@ -0,0 +1,7 @@
+findopengl-glu-include-dir
+--------------------------
+
+* The :module:`FindOpenGL` module learned to find a GLU include
+  directory different than the GL include directory.  A new
+  ``OPENGL_INCLUDE_DIRS`` result variable provides all include
+  directories.

Modules/FindOpenGL.cmake

@@ -92,12 +92,17 @@ This module sets the following variables:
  Defined if the system has GLES3.
 ``OPENGL_INCLUDE_DIR``
  Path to the OpenGL include directory.
+ The ``OPENGL_INCLUDE_DIRS`` variable is preferred.
 ``OPENGL_EGL_INCLUDE_DIRS``
  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.
  Clients are encouraged to use the ``OpenGL::*`` import targets instead.
+``OPENGL_INCLUDE_DIRS``
+  .. versionadded:: 3.29
+
+  Paths to the OpenGL include directories.
 
 .. versionadded:: 3.10
   Variables for GLVND-specific libraries ``OpenGL``, ``EGL`` and ``GLX``.
@@ -127,6 +132,11 @@ The following cache variables may also be set:
 
   Path to the OpenGL GLES3 library.
 
+``OPENGL_GLU_INCLUDE_DIR``
+  .. versionadded:: 3.29
+
+  Path to the OpenGL GLU include directory.
+
 .. versionadded:: 3.10
   Variables for GLVND-specific libraries ``OpenGL``, ``EGL`` and ``GLX``.
 
@@ -230,10 +240,13 @@ elseif (APPLE)
     "OpenGL GLU library")
   find_path(OPENGL_INCLUDE_DIR NAMES OpenGL/gl.h GL/gl.h DOC
     "Include for OpenGL")
+  find_path(OPENGL_GLU_INCLUDE_DIR NAMES OpenGL/glu.h GL/glu.h DOC
+    "Include for the OpenGL GLU library")
   list(APPEND _OpenGL_REQUIRED_VARS OPENGL_INCLUDE_DIR)
 
   list(APPEND _OpenGL_CACHE_VARS
     OPENGL_INCLUDE_DIR
+    OPENGL_GLU_INCLUDE_DIR
     OPENGL_gl_LIBRARY
     OPENGL_glu_LIBRARY
     )
@@ -289,6 +302,8 @@ else()
     /opt/graphics/OpenGL/include
   )
 
+  find_path(OPENGL_GLU_INCLUDE_DIR GL/glu.h ${_OPENGL_INCLUDE_PATH})
+
   list(APPEND _OpenGL_CACHE_VARS
     OPENGL_INCLUDE_DIR
     OPENGL_GLX_INCLUDE_DIR
@@ -296,6 +311,7 @@ else()
     OPENGL_GLES2_INCLUDE_DIR
     OPENGL_GLES3_INCLUDE_DIR
     OPENGL_xmesa_INCLUDE_DIR
+    OPENGL_GLU_INCLUDE_DIR
     )
 
   # Search for the GLVND libraries.  We do this regardless of COMPONENTS; we'll
@@ -509,7 +525,7 @@ else()
   set( OPENGL_XMESA_FOUND "NO" )
 endif()
 
-if(OPENGL_glu_LIBRARY)
+if(OPENGL_glu_LIBRARY AND (WIN32 OR OPENGL_GLU_INCLUDE_DIR))
   set( OPENGL_GLU_FOUND "YES" )
 else()
   set( OPENGL_GLU_FOUND "NO" )
@@ -567,6 +583,8 @@ unset(_OpenGL_REQUIRED_VARS)
 
 # OpenGL:: targets
 if(OPENGL_FOUND)
+  set(OPENGL_INCLUDE_DIRS ${OPENGL_INCLUDE_DIR})
+
   # ::OpenGL is a GLVND library, and thus Linux-only: we don't bother checking
   # for a framework version of this library.
   if(OPENGL_opengl_LIBRARY AND NOT TARGET OpenGL::OpenGL)
@@ -600,6 +618,7 @@ if(OPENGL_FOUND)
                           OpenGL::OpenGL)
     set_target_properties(OpenGL::GLX PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
                           "${OPENGL_GLX_INCLUDE_DIR}")
+    list(APPEND OPENGL_INCLUDE_DIRS ${OPENGL_GLX_INCLUDE_DIR})
   endif()
 
   # ::GLES2 is a GLVND library, and thus Linux-only: we don't bother checking
@@ -629,6 +648,7 @@ if(OPENGL_FOUND)
         INTERFACE_INCLUDE_DIRECTORIES
           "${OPENGL_GLES2_INCLUDE_DIR}"
     )
+    list(APPEND OPENGL_INCLUDE_DIRS ${OPENGL_GLES2_INCLUDE_DIR})
 
     if (OPENGL_USE_GLES2)
       set(_OpenGL_EGL_IMPL OpenGL::GLES2)
@@ -662,6 +682,7 @@ if(OPENGL_FOUND)
       INTERFACE_INCLUDE_DIRECTORIES
         "${OPENGL_GLES3_INCLUDE_DIR}"
     )
+    list(APPEND OPENGL_INCLUDE_DIRS ${OPENGL_GLES3_INCLUDE_DIR})
 
     if (OPENGL_USE_GLES3)
       set(_OpenGL_EGL_IMPL OpenGL::GLES3)
@@ -713,6 +734,7 @@ if(OPENGL_FOUND)
     # Note that EGL's include directory is different from OpenGL/GLX's!
     set_target_properties(OpenGL::EGL PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
                           "${OPENGL_EGL_INCLUDE_DIR}")
+    list(APPEND OPENGL_INCLUDE_DIRS ${OPENGL_EGL_INCLUDE_DIR})
   endif()
 
   if(OPENGL_GLU_FOUND AND NOT TARGET OpenGL::GLU)
@@ -727,6 +749,10 @@ if(OPENGL_FOUND)
     endif()
     set_target_properties(OpenGL::GLU PROPERTIES
       INTERFACE_LINK_LIBRARIES OpenGL::GL)
+    # Note that GLU's include directory may be different from OpenGL's!
+    set_target_properties(OpenGL::GLU PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                          "${OPENGL_GLU_INCLUDE_DIR}")
+    list(APPEND OPENGL_INCLUDE_DIRS ${OPENGL_GLU_INCLUDE_DIR})
   endif()
 
   # OPENGL_LIBRARIES mirrors OpenGL::GL's logic ...
@@ -743,6 +769,8 @@ if(OPENGL_FOUND)
   endif()
 endif()
 
+list(REMOVE_DUPLICATES OPENGL_INCLUDE_DIRS)
+
 # This deprecated setting is for backward compatibility with CMake1.4
 set(OPENGL_LIBRARY ${OPENGL_LIBRARIES})
 # This deprecated setting is for backward compatibility with CMake1.4

Tests/RunCMake/FindOpenGL/CMP0072-common.cmake

@@ -2,6 +2,7 @@ set(CYGWIN 0)
 set(WIN32 0)
 set(APPLE 0)
 set(OPENGL_INCLUDE_DIR GL/include)
+set(OPENGL_GLU_INCLUDE_DIR GLU/include)
 set(OPENGL_GLX_INCLUDE_DIR GLX/include)
 set(OPENGL_gl_LIBRARY GL)
 set(OPENGL_opengl_LIBRARY OpenGL)