Log Message:
databases/postgresql16: Fix krb5 defaults and dependencies.

Currently postgresql16's gssapi option requires mit-krb5.  It cannot
be safely linked, e.g. through libpq, into any application that also
links against a non-mit-krb5 libgssapi, such as heimdal as shipped in
NetBSD.

It is generally good to have gssapi support out of the box, since it
is probably the most widely deployed single-sign-on system on the
planet, and we've had it enabled by default for ages.  But it is now
incompatible with the default krb5 implementation in NetBSD and
pkgsrc on most platforms.  So, unless we can change pgsql to restore
heimdal support, this option has to be off unless you have switched
to mit-krb5 systemwide.

As discussed in this thread:

https://mail-index.netbsd.org/tech-pkg/2024/09/04/msg029866.html

Verified on NetBSD that:

- out of the box, gssapi is disabled by default
- with PKG_OPTIONS.postgresql16+=gssapi, PKG_FAIL_REASON fires
- with KRB5_DEFAULT=mit-krb5, gssapi is enabled by default

Also verified, with and without gssapi, that it builds a libpq.so
that does not link against /usr/lib/libgssapi.so (but with gssapi,
does link against mit-krb5's libgssapi_krb5.so).