Make the build reproducible
Whilst working on the Reproducible Builds effort [0] we noticed
that libnbd could not be built reproducibly.
This is due to it shipping a pod generation wrapper that
does not use/respect SOURCE_DATE_EPOCH [1] and additionally
varies the output depending on the build user's current
timezone.
(This was originally filed in Debian as #939546 [2].)
[0] https://reproducible-builds.org/
[1] https://reproducible-builds.org/docs/source-date-epoch/
[2] https://bugs.debian.org/939546
Signed-off-by: Chris Lamb <lamby@debian.org>