Configure glue for rxgk

Configure glue for rxgk

Add an --enable-rxgk switch to control whether the feature is used.
For the sake of buildbot coverage, we still attempt to build the core
subdirectory provided that a sufficiently usable GSS-API library
is available, but do not install anything when rxgk is disabled at
configure time.  Future commits will use the configure argument to
control the behavior of other rxgk-aware code in the tree.

We provide a few new symbols to conditionally compile code for rxgk.
The two new high-level symbols are:

- AFS_RXGK_ENV: when defined, rxgk is available
- AFS_RXGK_GSS_ENV: when defined, we can use GSS-API calls

AFS_RXGK_GSS_ENV is turned on only for userspace pthread builds. For
now, AFS_RXGK_ENV is only turned on for userspace pthread builds, and
non-ukernel kernel builds. This effectively disables rxgk integration
in any ukernel or LWP code, but this can be changed in the future by
changing when AFS_RXGK_ENV is defined.

Change-Id: Iab661d47aac77c1a238e809362015b869752df18
Reviewed-on: https://gerrit.openafs.org/10564
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>