Add support for make W=12, make W=123 and so on, to enable warnings from
multiple W= levels. Normally, make W=<level> does not include warnings
from the previous level.
Signed-off-by: NMichal Marek <mmarek@suse.cz>
Acked-by: NSam Ravnborg <sam@ravnborg.org>
Reviewed-By: NValdis Kletnieks <valdis.kletnieks@vt.edu>