Stop configuration when GCC is detected on macOS

cmake/Macros.cmake

@@ -23,9 +23,11 @@ function(sfml_set_stdlib target)
     if (SFML_OS_MACOSX)
         if (${CMAKE_GENERATOR} MATCHES "Xcode")
             sfml_set_xcode_property(${target} CLANG_CXX_LIBRARY "libc++")
-        else()
+        elseif(SFML_COMPILER_CLANG)
             target_compile_options(${target} PRIVATE "-stdlib=libc++")
             target_link_libraries(${target} PRIVATE "-stdlib=libc++")
+        else()
+            message(FATAL_ERROR "Clang is the only supported compiler on macOS")
         endif()
     endif()
 endfunction()