Add THREADS_ENABLED macro in order to compile Godot to run on the main thread

drivers/unix/os_unix.cpp

@@ -153,7 +153,9 @@ int OS_Unix::unix_initialize_audio(int p_audio_driver) {
 }
 
 void OS_Unix::initialize_core() {
+#ifdef THREADS_ENABLED
 	init_thread_posix();
+#endif
 
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_RESOURCES);
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_USERDATA);