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.
MFC After: 2 weeks
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