Fix Platform Thread Override

(cherry picked from commit 5fe1857496)

core/os/thread.cpp

@@ -28,9 +28,7 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-// Define PLATFORM_CUSTOM_THREAD_H in platform_config.h
-// Overriding the platform implementation is required in some proprietary platforms
-#ifndef PLATFORM_CUSTOM_THREAD_H
+#ifndef PLATFORM_THREAD_OVERRIDE // See details in thread.h
 
 #include "thread.h"
 
@@ -138,4 +136,4 @@ Thread::ID Thread::get_caller_id() {
 	}
 }
 #endif
-#endif // PLATFORM_CUSTOM_THREAD_H
+#endif // PLATFORM_THREAD_OVERRIDE