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 PR was done while working on reproducible builds for openSUSE.

[jpivarski]

This seems sensible; I can understand why you'd want the timestamp in the source code to not vary with time. The documentation on reproducible builds talk about reproducing the application binary, and both of these timestamps go into comments, which cannot affect the binaries, but I can see it being good to make it regular, just on principle.

I see that there's a fallback to time.time() if SOURCE_DATE_EPOCH is not defined, which is good. I don't know under what conditions the variable is defined (only during compilation?), but the fallback just reproduces the previous behavior, so I have no complaints.

Anyway, I'm happy with this PR and will merge it if I'm sure you're done. If you don't plan to make any more changes, let me know in a comment and I'll merge it into the codebase. Thanks!

[bmwiedemann]

Being able to trust binaries is the primary goal, but in the context of the openSUSE distribution, we also have python scripts and documentation all bundled into rpm packages and it is easier to reason for trust of our users if we are able to reproduce every bit in our packages and long term even the whole images.

[jpivarski]

@all-contributors please add @bmwiedemann for code

[allcontributors]

@jpivarski

I've put up a pull request to add @bmwiedemann! 🎉