Allow to override manpage date with SOURCE_DATE_EPOCH

Merged Bernhard M. Wiedemann requested to merge (removed):date into master

Allow to override man-page date with SOURCE_DATE_EPOCH in order 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.

Also use UTC via gmtime to be independent of timezone.

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