Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

drivers/unix/os_unix.cpp

@@ -35,7 +35,6 @@
 #include "core/config/project_settings.h"
 #include "core/debugger/engine_debugger.h"
 #include "core/debugger/script_debugger.h"
-#include "core/os/thread_dummy.h"
 #include "drivers/unix/dir_access_unix.h"
 #include "drivers/unix/file_access_unix.h"
 #include "drivers/unix/net_socket_posix.h"
@@ -63,6 +62,7 @@
 #include <string.h>
 #include <sys/time.h>
 #include <sys/wait.h>
+#include <time.h>
 #include <unistd.h>
 
 /// Clock Setup function (used by get_ticks_usec)
@@ -116,11 +116,10 @@ int OS_Unix::unix_initialize_audio(int p_audio_driver) {
 }
 
 void OS_Unix::initialize_core() {
-#ifdef NO_THREADS
-	ThreadDummy::make_default();
-#else
-	ThreadPosix::make_default();
+#if !defined(NO_THREADS)
+	init_thread_posix();
 #endif
+
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_RESOURCES);
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_USERDATA);
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_FILESYSTEM);