Warn at configure time about bitmap-later

It's superseded by DAFS and is slated for removal post-1.8.

Change-Id: Id2c870fb76eeb470bbf393e99654df434b1a1a86
Reviewed-on: http://gerrit.openafs.org/11525
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>