After src e26259f48afe, base no longer includes libgssapi, so remove
that from GSSAPILIBS.
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, arbitrarily pick a file which is
in kerberos-lib-dev and only exists with MIT: /usr/include/krad.h.