Allow to override build date

[bmwiedemann]

Allow to override build date with SOURCE_DATE_EPOCH.

See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of the environment variable.

Also uses gmtime/UTC to be independent of timezones.

This PR was done as part of my work on reproducible builds for openSUSE.

[brejoc]

@bmwiedemann Thanks! And thanks for the links with the explanation. Btw, is OBS already able to do reproducible builds?

[bmwiedemann]

yes, OBS is able to do reproducible and verifiable builds.
https://en.opensuse.org/openSUSE:Reproducible_Builds documents the 4 rpm macros you need in the prjconf to also have bit-identical rpm metadata.

https://build.opensuse.org/project/prjconf/Application:ERP:GNUHealth:Factory
already uses it, if you want to see how it looks.