Simplify GTK+ 2 version checks with a helper function

Use wx_is_at_least_gtk2(minor) instead of gtk_check_version(2, minor)
because it is more clear and also works as expected (i.e. returns true)
when using GTK+ 3 instead of creating difficult to notice bugs in the
code by failing the version check in this case.

See #18004.

src/gtk/display.cpp

@@ -35,7 +35,7 @@ static inline int wx_gdk_screen_get_primary_monitor(GdkScreen* screen)
 {
     int monitor = 0;
 #if GTK_CHECK_VERSION(2,20,0)
-    if (gtk_check_version(2,20,0) == NULL)
+    if (wx_is_at_least_gtk2(20))
         monitor = gdk_screen_get_primary_monitor(screen);
 #endif
     return monitor;