Cocoa: Fix ObjC being built as C with CMake 3.19

CMake 3.19 adds -xc when the LANGUAGE file property is C, breaking our
workaround for CMake 3.15 and earlier not understanding the .m suffix.

Fixes #1787.

src/CMakeLists.txt

@@ -112,7 +112,7 @@ target_include_directories(glfw PRIVATE
 target_link_libraries(glfw PRIVATE Threads::Threads ${glfw_LIBRARIES})
 
 # Workaround for CMake not knowing about .m files before version 3.16
-if (APPLE)
+if ("${CMAKE_VERSION}" VERSION_LESS "3.16" AND APPLE)
     set_source_files_properties(cocoa_init.m cocoa_joystick.m cocoa_monitor.m
                                 cocoa_window.m nsgl_context.m PROPERTIES
                                 LANGUAGE C)