Uses/gssapi.mk: Improve base Kerberos detection

We can't check for /usr/libexec/krb5kdc to determine if base uses
MIT Kerberos, because this is part of the kerberos-kdc package and
might not be installed. Instead, pick a pkgconfig file which is
only installed with MIT: /usr/libdata/pkgconfig/mit-krb5.pc.

Reviewed by: des, cy
Approved by: cy (ports)
Differential Revision: https://reviews.freebsd.org/D51841