Build: check for gencat

Check for the gencat program and give a missing warning if
not present.

Reviewed-on: http://gerrit.openafs.org/7203
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit dcefc4576020c2c076af1a4af07cc827bf4eea40)

Change-Id: I383af43e6bb26de0095629e99d41e0e0b6bcb685
Reviewed-on: http://gerrit.openafs.org/11158
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>