Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

Allow to override build date with SOURCE_DATE_EPOCH in order to make 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.

This patch was done while working on reproducible builds for openSUSE.

[praiskup]

Thank you for the patch. Note there's also the issue #75.

Since this patch is probably an RPM-only solution, I'd probably prefer having this date logic, note the latter detections should beat the former:

• time.time() by default
• SOURCE_DATE_EPOCH env var (solves RPM builds if no better detection available)
• what @ssbarnea suggests, i.e. detecting from .git
• the --date option (explicit decision)

The first two are implemented by this PR. The third can be added later, As such, I like the PR. @ssbarnea can you take a look?

[bmwiedemann]

SOURCE_DATE_EPOCH is set by various package builds, e.g. in Debian, ArchLinux, NixOS, Guix - in addition to rpm.

If you have a .git tree instead of a release tarball, you can also extract %ct from it.

An explicit --date option is not as useful, because it requires us to patch an infinite number of callers.

[praiskup]

Ok, thank you again for the patch!