Fixed compiler warning on 64-bit systems (but this is still broken, really).

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%403795

src/thread/pthread/SDL_systhread.c

@@ -99,7 +99,7 @@ void SDL_SYS_SetupThread(void)
 /* WARNING:  This may not work for systems with 64-bit pid_t */
 Uint32 SDL_ThreadID(void)
 {
-	return((Uint32)pthread_self());
+	return((Uint32)((size_t)pthread_self()));
 }
 
 void SDL_SYS_WaitThread(SDL_Thread *thread)