in order to allow for reproducible builds.
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.
Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>