Force Mach-O nasm format on Snow Leopard, something breaks in the detection
on that platform.
Also define KRB5_CONFIG for the case not using builtin mit-krb5.
Exactly disable krb5 if option is off, otherwise specify wanted krb5-config.

Bump PKGREVISION.
Updated www/squid3 to 3.4.3nb3
Limit PGSQL_VERSIONS_ACCEPTED to the same versions as those supported by
the main package.

Fixes problems with dependencies when using a postgresql version outside
those accepted.