newest VSI C compiler uses different predefined macro

recognize __vms predefined compiler macro and ensure that VMS
is also defined because that is what NetHack source files
currently expect.

include/global.h

@@ -166,6 +166,11 @@ typedef uchar nhsym;
  * Please don't change the order.  It does matter.
  */
 
+#ifdef __vms
+#ifndef VMS
+#define VMS
+#endif
+#endif
 #ifdef VMS
 #include "vmsconf.h"
 #endif