FindOpenGL: Provide imported targets for GL and GLU

Create OpenGL::GL and OpenGL::GLU imported targets using the locations
found.

This feature was originally added by commit v3.1.0-rc1~420^2~2
(FindOpenGL: Provide imported targets for GL and GLU, 2014-05-31) but
had to be reverted by commit v3.1.0-rc3~10^2 (FindOpenGL: Revert support
for imported targets, 2014-12-01) due to issue #15267.  Since then we
added support for `IMPORTED_LIBNAME` to interface libraries, so use it
to handle the case where we have only the library name without an
absolute path.

Inspired-by: Philipp Möller <bootsarehax@googlemail.com>
Closes: #15267

Help/release/dev/FindOpenGL-imported-targets.rst

@@ -0,0 +1,5 @@
+FindOpenGL-imported-targets
+---------------------------
+
+* The :module:`FindOpenGL` module now provides imported targets
+  ``OpenGL::GL`` and ``OpenGL::GLU`` when the libraries are found.

Modules/FindOpenGL.cmake

@@ -7,6 +7,16 @@
 #
 # FindModule for OpenGL and GLU.
 #
+# IMPORTED Targets
+# ^^^^^^^^^^^^^^^^
+#
+# This module defines the :prop_tgt:`IMPORTED` targets:
+#
+# ``OpenGL::GL``
+#  Defined if the system has OpenGL.
+# ``OpenGL::GLU``
+#  Defined if the system has GLU.
+#
 # Result Variables
 # ^^^^^^^^^^^^^^^^
 #
@@ -160,6 +170,55 @@ include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(OpenGL REQUIRED_VARS ${_OpenGL_REQUIRED_VARS})
 unset(_OpenGL_REQUIRED_VARS)
 
+# OpenGL:: targets
+if(OPENGL_FOUND)
+  if(NOT TARGET OpenGL::GL)
+    if(IS_ABSOLUTE "${OPENGL_gl_LIBRARY}")
+      add_library(OpenGL::GL UNKNOWN IMPORTED)
+      if(OPENGL_gl_LIBRARY MATCHES "/([^/]+)\\.framework$")
+        set(_gl_fw "${OPENGL_gl_LIBRARY}/${CMAKE_MATCH_1}")
+        if(EXISTS "${_gl_fw}.tbd")
+          set(_gl_fw "${_gl_fw}.tbd")
+        endif()
+        set_target_properties(OpenGL::GL PROPERTIES
+          IMPORTED_LOCATION "${_gl_fw}")
+      else()
+        set_target_properties(OpenGL::GL PROPERTIES
+          IMPORTED_LOCATION "${OPENGL_gl_LIBRARY}")
+      endif()
+    else()
+      add_library(OpenGL::GL INTERFACE IMPORTED)
+      set_target_properties(OpenGL::GL PROPERTIES
+        IMPORTED_LIBNAME "${OPENGL_gl_LIBRARY}")
+    endif()
+    set_target_properties(OpenGL::GL PROPERTIES
+      INTERFACE_INCLUDE_DIRECTORIES "${OPENGL_INCLUDE_DIR}")
+  endif()
+
+  if(OPENGL_GLU_FOUND AND NOT TARGET OpenGL::GLU)
+    if(IS_ABSOLUTE "${OPENGL_glu_LIBRARY}")
+      add_library(OpenGL::GLU UNKNOWN IMPORTED)
+      if(OPENGL_glu_LIBRARY MATCHES "/([^/]+)\\.framework$")
+        set(_glu_fw "${OPENGL_glu_LIBRARY}/${CMAKE_MATCH_1}")
+        if(EXISTS "${_glu_fw}.tbd")
+          set(_glu_fw "${_glu_fw}.tbd")
+        endif()
+        set_target_properties(OpenGL::GLU PROPERTIES
+          IMPORTED_LOCATION "${_glu_fw}")
+      else()
+        set_target_properties(OpenGL::GLU PROPERTIES
+          IMPORTED_LOCATION "${OPENGL_glu_LIBRARY}")
+      endif()
+    else()
+      add_library(OpenGL::GLU INTERFACE IMPORTED)
+      set_target_properties(OpenGL::GLU PROPERTIES
+        IMPORTED_LIBNAME "${OPENGL_glu_LIBRARY}")
+    endif()
+    set_target_properties(OpenGL::GLU PROPERTIES
+      INTERFACE_LINK_LIBRARIES OpenGL::GL)
+  endif()
+endif()
+
 mark_as_advanced(
   OPENGL_INCLUDE_DIR
   OPENGL_xmesa_INCLUDE_DIR

Tests/CMakeLists.txt

@@ -1400,6 +1400,10 @@ ${CMake_BINARY_DIR}/bin/cmake -DDIR=dev -P ${CMake_SOURCE_DIR}/Utilities/Release
     add_subdirectory(FindOpenCL)
   endif()
 
+  if(CMake_TEST_FindOpenGL)
+    add_subdirectory(FindOpenGL)
+  endif()
+
   if(CMake_TEST_FindOpenSSL)
     add_subdirectory(FindOpenSSL)
   endif()

Tests/FindOpenGL/CMakeLists.txt

@@ -0,0 +1,10 @@
+add_test(NAME FindOpenGL.Test COMMAND
+  ${CMAKE_CTEST_COMMAND} -C $<CONFIGURATION>
+  --build-and-test
+  "${CMake_SOURCE_DIR}/Tests/FindOpenGL/Test"
+  "${CMake_BINARY_DIR}/Tests/FindOpenGL/Test"
+  ${build_generator_args}
+  --build-project TestFindOpenGL
+  --build-options ${build_options}
+  --test-command ${CMAKE_CTEST_COMMAND} -V -C $<CONFIGURATION>
+  )

Tests/FindOpenGL/Test/CMakeLists.txt

@@ -0,0 +1,14 @@
+cmake_minimum_required(VERSION 3.7)
+project(TestFindOpenGL C)
+include(CTest)
+
+find_package(OpenGL REQUIRED)
+
+add_executable(test_tgt main.c)
+target_link_libraries(test_tgt OpenGL::GLU)
+add_test(NAME test_tgt COMMAND test_tgt)
+
+add_executable(test_var main.c)
+target_include_directories(test_var PRIVATE ${OPENGL_INGLUDE_DIRS})
+target_link_libraries(test_var PRIVATE ${OPENGL_LIBRARIES})
+add_test(NAME test_var COMMAND test_var)

Tests/FindOpenGL/Test/main.c

@@ -0,0 +1,17 @@
+#ifdef _WIN32
+#include <windows.h>
+#endif
+#ifdef __APPLE__
+#include <OpenGL/gl.h>
+#else
+#include <GL/gl.h>
+#endif
+
+#include <stdio.h>
+
+int main()
+{
+  /* Reference a GL symbol without requiring a context at runtime.  */
+  printf("&glGetString = %p\n", &glGetString);
+  return 0;
+}