define nhassert if not defined by port.

include/global.h

@@ -364,5 +364,9 @@ struct savefile_info {
 #define nethack_enter(argc, argv) ((void) 0)
 #endif
 
+/* Supply nhassert macro if not supplied by port */
+#ifndef nhassert
+#define nhassert(expression) ((void)0)
+#endif
 
 #endif /* GLOBAL_H */