Add some configure bits for GSS-API

rxgk will require gss_pseudo_random and might want a couple other
krb5-specific bits.  We'll also need substvars to tell whether or
not we can try building these things.

Change-Id: Id18eb3f554605875696095eb40c25ec54df1f74b
Reviewed-on: http://gerrit.openafs.org/10561
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>