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/bitmap.cpp

@@ -522,7 +522,7 @@ wxBitmap::wxBitmap(GdkPixmap* pixmap)
 wxBitmap::wxBitmap(const wxCursor& cursor)
 {
 #if GTK_CHECK_VERSION(2,8,0)
-    if (gtk_check_version(2,8,0) == NULL)
+    if (wx_is_at_least_gtk2(8))
     {
         GdkPixbuf *pixbuf = gdk_cursor_get_image(cursor.GetCursor());
         *this = wxBitmap(pixbuf);