Log Message:
Fix brokenness introduced as part of the update of zlib to 1.2.1.  The
updated package update bumped the zlib shared lib major, which required
that BUILDLINK_DEPENDS.zlib be bumped as well.  Rather then requiring
zlib>=1.2.1 for packages that can use the built-in zlib on *BSD systems,
allow those built-in versions to satisfy zlib>=1.1.4nb1 dependencies,
and only require the latest version if no suitable zlib is found.