Added nhassert() support.

util/lev_main.c

@@ -1637,4 +1637,11 @@ short ospeed;
 #endif
 #endif /* STRICT_REF_DEF */
 
+/* nhassert_failed is called when an nhassert's condition is false */
+void nhassert_failed(const char * exp, const char * file, int line)
+{
+    fprintf(stderr, "NHASSERT(%s) in '%s' at line %d\n", exp, file, line);
+    exit(EXIT_FAILURE);
+}
+
 /*lev_main.c*/