Make man page date reproducible

[bmwiedemann]

Make man page date reproducible

See https://reproducible-builds.org/ for why this is good

CMake already uses SOURCE_DATE_EPOCH for TIMESTAMP
The SOURCE_DATE_EPOCH variable is documented in
https://reproducible-builds.org/docs/source-date-epoch/

Also use UTC to be independent of timezone.

This is mostly a rebase of #553 that I finally managed to test on 0.9.0 now.

This PR was done while working on reproducible builds for openSUSE.

[t-wissmann]

Thanks for fixing it! Sorry that I forgot about your original PR after winterbreeze had been merged.

Am I correct, that now, package maintainers can set BUILD_DATE manually to achieve a reproducable build? By the way, for the man pages in released versions, the man page date can also be taken from the first line matching Release.* on .* and only for git versions we need to improvise (e.g. take a TIMESTAMP or look up the git log)

[bmwiedemann]

maintainers can set SOURCE_DATE_EPOCH and cmake will use that to fill BUILD_DATE.
rpmbuild and Debian's and ArchLinux' package build automatically set SOURCE_DATE_EPOCH to the latest package changelog entry date, so it can be reproducible by default there.

For reproducible git builds, you could set it to git log -1 --format=%ct

[codecov]

Codecov Report

Merging #1029 into master will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1029   +/-   ##
======================================
  Coverage    90.0%   90.0%           
======================================
  Files         102     102           
  Lines        8633    8633           
======================================
+ Hits         7777    7778    +1     
+ Misses        856     855    -1