Allow to override the build date

using the SOURCE_DATE_EPOCH env var
to make vpp package 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.

Change-Id: Iae0935c9bb4ccdfdb65600be543945a88f2e137f
Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>