Log Message:
Move libexec/squid_unix_group and man/man8/squid_unix_group.8 from PLIST
to OPTIONAL_FILES in Makefile.

This fix a problem when setting SQUID_CONFIGURE_ARGS in /etc/mk.conf
without --enable-external-acl-helpers=unix_group.

Noted by private mail from Tomasz Luchowski <tomasz at luchowski.com>.