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

3 files changed, 41 insertions, 0 deletions

diff --git a/Tests/FindOpenGL/CMakeLists.txt b/Tests/FindOpenGL/CMakeLists.txt
new file mode 100644
index 0000000..9aa3aba
--- /dev/null
+++ b/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>
+  )
diff --git a/Tests/FindOpenGL/Test/CMakeLists.txt b/Tests/FindOpenGL/Test/CMakeLists.txt
new file mode 100644
index 0000000..cac3424
--- /dev/null
+++ b/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)
diff --git a/Tests/FindOpenGL/Test/main.c b/Tests/FindOpenGL/Test/main.c
new file mode 100644
index 0000000..bca3d7e
--- /dev/null
+++ b/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;
+}