Fix --without-krb5

Fix --without-krb5

Currently, specifying --without-krb5 causes the AM_CONDITIONAL
KRB5_USES_COM_ERR to not be defined, which makes configure refuse to
run successfully. Fix this by forcing KRB5_USES_COM_ERR to always be
false if we are running explicitly without krb5.

Reviewed-on: http://gerrit.openafs.org/4576
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 9b405a7b082a8498fb6e918df73482ad05e251ea)

Change-Id: I4e706a828e205cf649f09d770986bdfb27f984aa
Reviewed-on: http://gerrit.openafs.org/11636
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>