To allow people to locally build bit-identical packages to those coming from
OBS, 3 macros are needed in prjconf, but only the 1st is already enabled
in Factory (and Leap 15.0)

Macros:
%source_date_epoch_from_changelog Y
%clamp_mtime_to_source_date_epoch Y
%use_source_date_epoch_as_buildtime Y
# optional:
%_buildhost reproducible


I hope, we can discuss/test/enable these until 2020
so that it gets enough coverage before Leap 16

Normalizing rpm metadata was known to have some negative effects.
For python3, this should be good now with
https://www.python.org/dev/peps/pep-0552/ in python3.7 that reached Tumbleweed
in 2019-02.
For python2 we still carry my older patch that solves it by normalizing the
.pyc file content.

There is no hurry at the moment, because the benefit of bit-identical over
mostly-the-same is small and some risks and trade-offs are involved.

https://build.suse.de/project/show/SUSE:Factory:Head
already has 3 macros enabled, and allows for bit-identical local builds.

https://en.opensuse.org/openSUSE:Reproducible_Builds has more details.