Define __WXGTK220__ in configure and improve checks for GTK+ version.

Use AC_CACHE_CHECK() to avoid recompiling the test program(s) every time
configure runs and also simplify the checks. This required renaming
ac_wxgtkXXX to wx_cv_gtkXXX (cached variables must have "cv" in them).

Also make the checks more logical, by simply setting wx_cv_gtkXXX if
wx_cv_gtkYYY is set with XXX < YYY.

Finally, and the real reason for these changes, add test for __WXGTK220__
which will be used by the upcoming commits to check whether GtkSpinner is
available.

setup.h_vms

@@ -101,6 +101,9 @@
 /* Define this if your version of GTK+ is greater than 2.18 */
 #undef __WXGTK218__
 
+/* Define this if your version of GTK+ is greater than 2.20 */
+#undef __WXGTK220__
+
 #include <types.h>
 typedef pid_t GPid;