Polish Kerberos PAC detection (#2330)

Polish Kerberos PAC detection (#2330)

PAC is only used with GSSAPI, no need to detect unless GSSAPI is
also found to be working.

Shuffle build-time predefine logic into autoconf to set
HAVE_KRB5_PAC_SUPPORT when needed.

AC_CHECK_FUNC automatically defines the HAVE_function. No need
to do that ourselves.

Also fixes unused HAVE_GSS_MAP_ANY_TO_ANY which looks like a
typo of the tested *_many_to_any().