build: Make sure SOURCE_DATE_EPOCH is in the past

[bmwiedemann]

Make sure SOURCE_DATE_EPOCH is in the past,
otherwise, builds before noon will not have normalized mtimes
from %clamp_mtime_to_source_date_epoch

This also helps other programs like tar --clamp-mtime

Tested successfully on openSUSE.
To reproduce, create a changelog entry with only a date

* Fri Aug 31 2018 user@host
- dummy

and build it before 12:00 UTC
using macros

%source_date_epoch_from_changelog Y
%clamp_mtime_to_source_date_epoch Y
%use_source_date_epoch_as_buildtime Y
%_buildhost reproducible

rpm -qpvl showed file mtime values to not be normalized
and therefore build results were not bit-identical.

Edit: would be nice to avoid the modulo heuristic and find out if there was a time in the changelog or just a date. How?

[bmwiedemann]

I'm already producing bit-identical RPMs using the 4 macros documented in
https://en.opensuse.org/openSUSE:Reproducible_Builds
So no changes on digests or signatures are needed atm.

Inaccuracy of $SOURCE_DATE_EPOCH does not matter, as long as it is in the past. Some people always set it to 1 (aka 1970-01-01 00:00:01) and are happy.