Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

Allow to override build date with SOURCE_DATE_EPOCH 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 date call works with different variants of date (GNU, FreeBSD, ...).

Also use UTC to be independent of timezone.

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

[rtmclay]

Thanks for this PR. I am happy to let the build date for Lmod to be set to UTC when the variable SOURCE_DATE_EPOCH defined. Otherwise, I prefer that the build time remain the way it has been.

I have updated the testing branch to have a new proj_mgmt/DATE_cmd.sh. Please try the testing branch to see if it works for you.

[bmwiedemann]

I tested that your version (commit 2077924) builds reproducibly.

The line 2077924#diff-2c6b6228189aa8847f547868c4dcd1795539aca42b27032544b3f00f1a35b74cR18 I don't understand.

If it makes the code simpler / more maintainable we can also omit the UTC (-u) part completely, then callers will just have to ensure a normalized timezone env. We already do that in openSUSE.

Edit: I pushed this change to the PR now.

[rtmclay]

The line you asked about just removes "%:z" and replaces it with "%Z". This means that the date-time has UTC and not +00:00.

I am ready to close this PR