FindOpenGL: Add support for GLVND on Linux

Find GLVND components if available.  Add `GLX` and `EGL` options for
COMPONENTS that allow requesting these libraries explicitly.  Introduce
new import targets for these windowing-system-specific libraries.

On a GLVND system, populate the legacy `OPENGL_LIBRARIES` variable and
the `OpenGL::GL` target using the `OpenGL` and `GLX` components.  On
non-GLVND systems, continue to use the legacy `GL` library and simply do
not provide the GLVND components.  Application code can choose to adapt
based on the availability of GLVND components as imported targets.

1 files changed, 57 insertions, 1 deletions

diff --git a/Tests/FindOpenGL/Test/CMakeLists.txt b/Tests/FindOpenGL/Test/CMakeLists.txt
index cac3424..3b5ffee 100644
--- a/Tests/FindOpenGL/Test/CMakeLists.txt
+++ b/Tests/FindOpenGL/Test/CMakeLists.txt
@@ -1,14 +1,70 @@
-cmake_minimum_required(VERSION 3.7)
+cmake_minimum_required(VERSION 3.9)
 project(TestFindOpenGL C)
 include(CTest)
 
 find_package(OpenGL REQUIRED)
 
+# import target for GLU
 add_executable(test_tgt main.c)
 target_link_libraries(test_tgt OpenGL::GLU)
 add_test(NAME test_tgt COMMAND test_tgt)
 
+# OPENGL_LIBRARIES should be whatever libraries are needed to link.
 add_executable(test_var main.c)
 target_include_directories(test_var PRIVATE ${OPENGL_INGLUDE_DIRS})
 target_link_libraries(test_var PRIVATE ${OPENGL_LIBRARIES})
 add_test(NAME test_var COMMAND test_var)
+
+# VND support adds an ::OpenGL import target.  This can be used for OpenGL-only
+# code (code that does not manipulate contexts, like our 'main.c').  Without
+# VND, ::GL can be used for both context and non-context OpenGL code.
+if(OpenGL_TEST_VND)
+  add_executable(test_comp_none main.c)
+  target_link_libraries(test_comp_none PRIVATE OpenGL::OpenGL)
+  add_test(NAME test_comp_none COMMAND test_comp_none)
+else()
+  add_executable(test_comp_none main.c)
+  target_link_libraries(test_comp_none PRIVATE OpenGL::GL)
+  add_test(NAME test_comp_none COMMAND test_comp_none)
+endif()
+
+# GLX
+if(OpenGL_TEST_VND)
+  find_package(OpenGL REQUIRED COMPONENTS OpenGL GLX)
+  add_executable(test_comp_glx main.c)
+  target_link_libraries(test_comp_glx PRIVATE OpenGL::OpenGL OpenGL::GLX)
+  add_test(NAME test_comp_glx COMMAND test_comp_glx)
+else()
+  # non-VND systems won't have it, but an optional search for GLX should still
+  # be okay.
+  find_package(OpenGL COMPONENTS GLX)
+  add_executable(test_comp_glx_novnd main.c)
+  target_link_libraries(test_comp_glx_novnd PRIVATE OpenGL::GL)
+  add_test(NAME test_comp_glx_novnd COMMAND test_comp_glx_novnd)
+endif()
+
+# EGL is only available on Linux+GLVND at present.
+if(OpenGL_TEST_VND)
+  find_package(OpenGL COMPONENTS OpenGL EGL)
+  if(OpenGL_EGL_FOUND)
+    add_executable(test_comp_egl main.c)
+    target_link_libraries(test_comp_egl PRIVATE OpenGL::OpenGL OpenGL::EGL)
+    add_test(NAME test_comp_egl COMMAND test_comp_egl)
+    # EGL-only code should not link to GLX.
+    execute_process(COMMAND ldd test_comp_egl
+                    OUTPUT_VARIABLE LDD_OUT
+                    ERROR_VARIABLE LDD_ERR)
+    if("${LDD_OUT}" MATCHES "GLX")
+      message(FATAL_ERROR "EGL-only code links to GLX!")
+    endif()
+  endif()
+
+  # all three COMPONENTS together.
+  find_package(OpenGL COMPONENTS OpenGL EGL GLX)
+  if(OpenGL_EGL_FOUND AND OpenGL_GLX_FOUND)
+    add_executable(test_comp_both main.c)
+    target_link_libraries(test_comp_both PRIVATE OpenGL::OpenGL OpenGL::EGL
+                          OpenGL::GLX)
+    add_test(NAME test_comp_both COMMAND test_comp_both)
+  endif()
+endif()