external: use GCC-specific flags only for GCC/Clang.

external/OpenGL/GL/CMakeLists.txt

@@ -23,6 +23,11 @@
 #
 
 add_library(MagnumGLLoadGenObjects OBJECT gl_magnum.c)
-set_target_properties(MagnumGLLoadGenObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS} -fvisibility=hidden -DGLLoadGen_EXPORTS")
+
+if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+    set_target_properties(MagnumGLLoadGenObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS} -fvisibility=hidden -DGLLoadGen_EXPORTS")
+else()
+    set_target_properties(MagnumGLLoadGenObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS} -DGLLoadGen_EXPORTS")
+endif()
 
 install(FILES gl_magnum.h DESTINATION ${MAGNUM_INCLUDE_INSTALL_DIR}/OpenGL/GL)