longfin missing gssapi_ext.h

Greetings,

Looks like longfin has a particularly old Kerberos/GSSAPI installation
on it which pre-dates MIT release 1.11 from circa 2012 and is missing
gssapi_ext.h, causing the recently committed patch to add Kerberos
credential delegation to fail to build.

I'm inclined to update our configure check to explicitly check for the
needed function (gss_store_cred_into) as no one should really be running
with such an out-dated (over a decade old...) version of MIT Kerberos.

Thoughts?

Thanks!

Stephen