lsg.c: Make the output of lopsubgen reproducible.

Whilst working on the Reproducible Builds effort [0], we noticed that
liblopsub generates output that is not reproducible. The lopsubgen
utility does not respect SOURCE_DATE_EPOCH [1] and thus packages such
as src:tfortune are rendered unreproducible as they then encode the
build date and time.

This patch makes lopsubgen honour SOURCE_DATE_EPOCH.

[0] https://reproducible-builds.org/
[1] https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Andre Noll <maan@tuebingen.mpg.de>