AFS_component_version_number.c: Respect SOURCE_DATE_EPOCH if set

AFS_component_version_number.c: Respect SOURCE_DATE_EPOCH if set

To improve build reproducibility, if the SOURCE_DATE_EPOCH environment
variable is set, use it to deterministically replace the embedded build
date, and do not include the username or hostname in this case.

https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal

Change-Id: I9ba951f1836385ffd14aad95f071bf8c672a01bb
Reviewed-on: https://gerrit.openafs.org/12471
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>