(define_makeflags): Terminate properly when FLAGSTRING is empty.

main.c

@@ -1781,6 +1781,8 @@ define_makeflags (all, makefile)
 	  p += sizeof ref - 1;
 	}
     }
+  else if (p == &flagstring[1])
+    --p;
   else if (p[-1] == '-')
     /* Kill the final space and dash.  */
     p -= 2;