Added SDL_DetachThread() API.

src/thread/generic/SDL_systhread.c

@@ -62,4 +62,10 @@ SDL_SYS_WaitThread(SDL_Thread * thread)
     return;
 }
 
+void
+SDL_SYS_DetachThread(SDL_Thread * thread)
+{
+    return;
+}
+
 /* vi: set ts=4 sw=4 expandtab: */