Resync src/cf/gssapi.m4 with rra-c-util 3.3 (fixes OpenBSD)

Resync src/cf/gssapi.m4 with rra-c-util 3.3 (fixes OpenBSD)

Resynchronize src/cf/gssapi.m4 with rra-c-util 3.3.  This includes
the following fixes:

* Handle the Heimdal GSS-API library on OpenBSD, which does not have
  a separate libroken.  Thanks to Antoine Verheijen for the analysis.

* Search /usr/kerberos/bin for krb5-config even if that isn't on the
  user's PATH to find krb5-config on some Red Hat versions.

* Use [] to mark empty arguments to Autoconf macros.

* Add an explicit license (matching the normal Autoconf macro license).

Change-Id: I6934ebcb3bfb0d763713aef159484b08849d4985
Reviewed-on: http://gerrit.openafs.org/4587
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>