Broken _DEPENDS logic

[Roger Marquis]

Found why mailman is trying (and failing) to reinstall postfix and it
appears to be a bug in other ports as well.

   # cd /usr/ports/mail && grep '_DEPENDS+=.*postfix' */Makefile
   dk-milter/Makefile:RUN_DEPENDS+= ${LOCALBASE}/libexec/postfix/smtpd:${PORTSDIR}/mail/postfix-current
   dk-milter/Makefile:RUN_DEPENDS+=        ${LOCALBASE}/libexec/postfix/smtpd:${PORTSDIR}/mail/postfix
   drac/Makefile:RUN_DEPENDS+=     postfix:${PORTSDIR}/mail/postfix
   enma/Makefile:RUN_DEPENDS+=     ${LOCALBASE}/libexec/postfix/smtpd:${PORTSDIR}/mail/postfix-current
   enma/Makefile:RUN_DEPENDS+=     ${LOCALBASE}/libexec/postfix/smtpd:${PORTSDIR}/mail/postfix
   mailman/Makefile:BUILD_DEPENDS+=        /usr/sbin/postconf:${PORTSDIR}/mail/postfix
   mailman/Makefile:RUN_DEPENDS+=  /usr/sbin/postconf:${PORTSDIR}/mail/postfix

Shouldn't these ports be querying the pkg db rather than checking for a
particular file, particularly when the file is incorrectly specified?

Roger