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.

Kind: documentation / bug fix / feature / performance / infrastructure.

Fixes / closes #11037

• Added / updated test-suite

• Corresponding documentation was added / updated (including any warning and error messages added / removed / modified).
• Entry added in the changelog (see https://github.com/coq/coq/tree/master/doc/changelog#unreleased-changelog for details).

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

[ejgallego]

Indeed, this does not work as noted by Gaëtan.

[ejgallego]

Thanks for the patch @bmwiedemann , it looks good to me!

IMHO we should document this variable in configure -help, so I propose you add a brief documentation and fix the indentation while you are at it.

[bmwiedemann]

I don't think it should go into configure -help output, because people doing reproducible builds already know very well about this variable, because it is used for gcc, rpm and so many more tools.
And other people will probably not need it.

But if you still want it, I will not be unhappy. I would just need some help finding the right place to patch.

[ejgallego]

I don't think it should go into configure -help output, because people doing reproducible builds already know very well about this variable, because it is used for gcc, rpm and so many more tools.

@bmwiedemann your call, you are the expert here. I am fine merging as is; if you want to add some documentation the easiest would be to tweak the "Available options" message parse_args () = ...

I certainly didn't know about the variable even if I am a bit familiar with RB.

[Zimmi48]

Here or elsewhere (e.g. INSTALL file) but that Coq supports this absolutely needs to be documented beyond the changelog.

[bmwiedemann]

@Zimmi48 added some lines to INSTALL. Can you think of a better way to place/word it?

[Zimmi48]

This wording does look pretty good to me.

[bmwiedemann]

Thanks for your quick and helpful feedback.

For the remaining other issue I opened #11229