gtx: add configure switch to force not building

gtx: add configure switch to force not building

allow gtx and associated software to be turned off.

Reviewed-on: http://gerrit.openafs.org/7552
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 5d53c12b95c6ffac6c00e4fec6138a51b6185dd7)

Change-Id: I79e3e2116dd244f4561be841beecb7c24eb59a18
Reviewed-on: http://gerrit.openafs.org/12095
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>