Post AppDefined events from the main thread ONLY (bug#23934)

* src/nsterm.h: Make nextappdefined var not just GNUStep.
* src/nsterm.c (ns_send_appdefined, sendFromMainThread): Remove GNUStep

src/nsterm.h

@@ -380,9 +380,9 @@ char const * nstrace_fullscreen_type_name (int);
 #endif
 #ifdef NS_IMPL_GNUSTEP
   BOOL applicationDidFinishLaunchingCalled;
+#endif
 @public
   int nextappdefined;
-#endif
 }
 - (void)logNotification: (NSNotification *)notification;
 - (void)antialiasThresholdDidChange:(NSNotification *)notification;

src/nsterm.m

@@ -3918,8 +3918,8 @@ overwriting cursor (usually when cursor on a tab) */
 {
   NSTRACE_WHEN (NSTRACE_GROUP_EVENTS, "ns_send_appdefined(%d)", value);
 
-#ifdef NS_IMPL_GNUSTEP
   // GNUstep needs postEvent to happen on the main thread.
+  // Cocoa needs nextEventMatchingMask to happen on the main thread too.
   if (! [[NSThread currentThread] isMainThread])
     {
       EmacsApp *app = (EmacsApp *)NSApp;
@@ -3929,7 +3929,6 @@ overwriting cursor (usually when cursor on a tab) */
                          waitUntilDone:YES];
       return;
     }
-#endif
 
   /* Only post this event if we haven't already posted one.  This will end
        the [NXApp run] main loop after having processed all events queued at
@@ -5542,12 +5541,10 @@ - (void)timeout_handler: (NSTimer *)timedEntry
   ns_send_appdefined (-2);
 }
 
-#ifdef NS_IMPL_GNUSTEP
 - (void)sendFromMainThread:(id)unused
 {
   ns_send_appdefined (nextappdefined);
 }
-#endif
 
 - (void)fd_handler:(id)unused
 /* --------------------------------------------------------------------------