LU-3490 kerberos: Enable GSSAPI support by default

LU-3490 kerberos: Enable GSSAPI support by default

This change enables GSSAPI support by default when its prerequisites
are available.  Without this change, the GSSAPI code does not get
tested when commits are made to Gerrit, even if the prerequisite
GSSAPI and Kerberos libraries are installed.

Signed-off-by: Andrew Korty <ajk@iu.edu>
Change-Id: If661cc6bb1188f3678f70189d5e333219c31052d
Reviewed-on: http://review.whamcloud.com/6740
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Ken Hornstein <kenh@cmf.nrl.navy.mil>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>