[SV 64115] Suppress warnings about undefined GNUMAKEFLAGS

* src/variable.c (defined_vars): Add GNUMAKEFLAGS to defined variables.
* tests/scripts/options/warn: Add a test.

src/variable.c

@@ -1916,6 +1916,7 @@ static const struct defined_vars defined_vars[] = {
   { STRING_SIZE_TUPLE ("VPATH") },
   { STRING_SIZE_TUPLE ("GPATH") },
   { STRING_SIZE_TUPLE (WARNINGS_NAME) },
+  { STRING_SIZE_TUPLE (GNUMAKEFLAGS_NAME) },
   { NULL, 0 }
 };