Make pam conditional on INSTALL_KAUTH

Make pam conditional on INSTALL_KAUTH

The pam module we provide is only useful in kaserver-like environments,
and as such should not be installed when the user has requested to
not have kauth.

Change-Id: I9b336593e34cedfd6e8c2210f3798575d115d2d6
Reviewed-on: http://gerrit.openafs.org/11466
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>