Initialize X11 threading when Qt threading is enabled

This closes Bug 812
(cherry picked from commit 1f39650292)

src/kernel/qapplication_x11.cpp

@@ -1632,6 +1632,12 @@ void qt_init_internal( int *argcptr, char **argv,
     setlocale( LC_ALL, "" );		// use correct char set mapping
     setlocale( LC_NUMERIC, "C" );	// make sprintf()/scanf() work
 
+#if defined(QT_THREAD_SUPPORT)
+    if ( qt_is_gui_used ) {
+	XInitThreads();
+    }
+#endif
+
     if ( display ) {
 	// Qt part of other application