Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

Allow to override build date with SOURCE_DATE_EPOCH to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This date call works with different flavors of date (GNU, BSD, other).
Alternatively the date could be dropped completely to simplify the code.

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

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.32%. Comparing base (1827713) to head (bd93270).
Report is 85 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1435   +/-   ##
=======================================
  Coverage   65.32%   65.32%           
=======================================
  Files          50       50           
  Lines        8763     8763           
  Branches     1034     1034           
=======================================
  Hits         5724     5724           
  Misses       3039     3039           

Flag	Coverage Δ
unittests	65.32% <ø> (ø)

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[bynect]

The date is not really necessary but it was nice to have.

I understand the need for reproducible builds, so I approve these changes.

If possible I would like to keep support for BSD.

[bmwiedemann]

I updated the patch to also support BSD.