Add OpenGL::GL cmake imported targets (#23705)

This allows projects to include OpenGL like so
```cmake
find_package(OpenGL REQUIRED)
target_link_libraries(myproject OpenGL::GL)
```
Link to CMake's documentation:
https://cmake.org/cmake/help/latest/module/FindOpenGL.html#imported-targets

CMake's `FindOpenGL.cmake` also produces GLES-related imported targets.
While I think they would be nice to add, I've omitted them as I don't
know if it would be as simple as duplicating the `OpenGL::GL` target or
more needs to be to communicate that choice of flavor to emscripten.

Author: cajomar

cmake/Modules/FindOpenGL.cmake

@@ -30,3 +30,15 @@ mark_as_advanced(
   OPENGL_glu_LIBRARY
   OPENGL_gl_LIBRARY
 )
+
+add_library(OpenGL::GL INTERFACE IMPORTED)
+set_target_properties(OpenGL::GL PROPERTIES
+  IMPORTED_LIBNAME "${OPENGL_gl_LIBRARY}"
+  INTERFACE_INCLUDE_DIRECTORIES "${OPENGL_INCLUDE_DIR}"
+)
+
+add_library(OpenGL::GLU INTERFACE IMPORTED)
+set_target_properties(OpenGL::GLU PROPERTIES
+  IMPORTED_LIBNAME "${OPENGL_glu_LIBRARY}"
+  INTERFACE_LINK_LIBRARIES OpenGL::GL
+)

test/cmake/find_modules/CMakeLists.txt

@@ -6,15 +6,21 @@ add_executable(test_prog test.c)
 find_package(OpenGL REQUIRED)
 include_directories(${OPENGL_INCLUDE_DIR})
 target_link_libraries(test_prog ${OPENGL_LIBRARIES})
-message(" test: OPENGL_LIBRARIES: ${OPENGL_LIBRARIES}")
+message(STATUS " test: OPENGL_LIBRARIES: ${OPENGL_LIBRARIES}")
 
 find_package(OpenAL REQUIRED)
 include_directories(${OPENAL_INCLUDE_DIR})
 target_link_libraries(test_prog ${OPENAL_LIBRARY})
-message(" test: OPENGL_LIBRARIES: ${OPENAL_LIBRARIES}")
+message(STATUS " test: OPENAL_LIBRARIES: ${OPENAL_LIBRARIES}")
 
 find_package(SDL2 REQUIRED)
 include_directories(${SDL2_INCLUDE_DIRS})
 target_link_libraries(test_prog SDL2::SDL2)
-message(" test: SDL2_LIBRARIES: ${SDL2_LIBRARIES}")
-message(" test: SDL2_INCLUDE_DIRS: ${SDL2_INCLUDE_DIRS}")
+message(STATUS " test: SDL2_LIBRARIES: ${SDL2_LIBRARIES}")
+message(STATUS " test: SDL2_INCLUDE_DIRS: ${SDL2_INCLUDE_DIRS}")
+
+get_target_property(TEST_OPENGL_GL_LIBNAME OpenGL::GL IMPORTED_LIBNAME)
+message(STATUS " test: OpenGL::GL IMPORTED_LIBNAME: ${TEST_OPENGL_GL_LIBNAME}")
+
+get_target_property(TEST_OPENGL_GL_INCLUDES OpenGL::GL INTERFACE_INCLUDE_DIRECTORIES)
+message(STATUS " test: OpenGL::GL INTERFACE_INCLUDE_DIRECTORIES: ${TEST_OPENGL_GL_INCLUDES}")

test/cmake/find_modules/test.c

@@ -1,4 +1,4 @@
-// Invlude emscripten/version.h to ensure that the in-tree
+// Include emscripten/version.h to ensure that the in-tree
 // include directory has not been added to the include path.
 #include <emscripten/version.h>
 #include <stdio.h>

test/test_other.py

@@ -1091,7 +1091,9 @@ def test_cmake_install(self):
 
   @requires_network
   def test_cmake_find_modules(self):
-    self.run_process([EMCMAKE, 'cmake', test_file('cmake/find_modules')])
+    output = self.run_process([EMCMAKE, 'cmake', test_file('cmake/find_modules')], stdout=PIPE).stdout
+    self.assertContained(' test: OpenGL::GL IMPORTED_LIBNAME: GL', output)
+    self.assertContained(' test: OpenGL::GL INTERFACE_INCLUDE_DIRECTORIES: /.+/cache/sysroot/include', output, regex=True)
     self.run_process(['cmake', '--build', '.'])
     output = self.run_js('test_prog.js')
     self.assertContained('AL_VERSION: 1.1', output)