fix crashes in wxGTK3 when running with non-X11 backend, see #16688

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78167 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/private/gtk2-compat.h

@@ -517,5 +517,9 @@ static inline void wx_gtk_widget_get_preferred_size(GtkWidget* widget, GtkRequis
 }
 #define gtk_widget_get_preferred_size wx_gtk_widget_get_preferred_size
 
+// There is no equivalent in GTK+ 2, but it's not needed there anyhow as the
+// backend is determined at compile time in that version.
+#define GDK_IS_X11_DISPLAY(dpy) true
+
 #endif // !__WXGTK3__
 #endif // _WX_GTK_PRIVATE_COMPAT_H_