Modernize Semaphore

- Based on C++11's `mutex` and `condition_variable`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`

platform/server/os_server.cpp

@@ -68,10 +68,6 @@ void OS_Server::initialize_core() {
 	crash_handler.initialize();
 
 	OS_Unix::initialize_core();
-
-#ifdef __APPLE__
-	SemaphoreOSX::make_default();
-#endif
 }
 
 Error OS_Server::initialize(const VideoMode &p_desired, int p_video_driver, int p_audio_driver) {