LibVNCServer: fix starting of an onHold-client in threaded mode.

Discovered by madscientist159 on 11 Jan 2015:

"noted in testing with the threaded server build, whereby
if newClientHook() returned RFB_CLIENT_ON_HOLD there was no
way to release the hold when the server became ready"

libvncserver/main.c

@@ -612,21 +612,18 @@ listenerRun(void *data)
     return(NULL);
 }
 
-void 
-rfbStartOnHoldClient(rfbClientPtr cl)
-{
-    pthread_create(&cl->client_thread, NULL, clientInput, (void *)cl);
-}
-
-#else
+#endif
 
 void 
 rfbStartOnHoldClient(rfbClientPtr cl)
 {
-	cl->onHold = FALSE;
+    cl->onHold = FALSE;
+#ifdef LIBVNCSERVER_HAVE_LIBPTHREAD
+    if(cl->screen->backgroundLoop)
+	pthread_create(&cl->client_thread, NULL, clientInput, (void *)cl);
+#endif
 }
 
-#endif
 
 void 
 rfbRefuseOnHoldClient(rfbClientPtr cl)