Allow to override build date with SOURCE_DATE_EPOCH

Bernhard M. Wiedemannrequested to merge
bmwiedemann/ntpsec:date into master

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 patch was done while working on reproducible builds for openSUSE.

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