mk: WITH_FOO=no now generates a warning

Many people are used to gnu configure's behavior of changing
--with-foo=no to --without-foo. At the same time, several folks have
WITH_FOO=no in their config files to enable this ironic form of the
option because of an old meme from IRC, a mailing list or the forums (I
forget which). Add a warning to allow to alert people w/o breaking POLA.

Reviewed by: allanjude, bdrewery, manu
MFC After: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30684