Log Message:
Force use of pkgsrc libmilter, not builtin.  If the builtin version is 8.12
(as with NetBSD 2, for instance), but pkgsrc sendmail 8.13 is installed,
then clamav will attempt to use the 8.13 milter API, and fail linking.
(It probably should use an autoconf symbol test instead.)

This change forces an API at least new enough to match the latest version
offered via pkgsrc; and since libmilter is a static library, it still
ends up with no runtime DEPENDS.

No PKGREVISION bump required, as milter is a non-default option.