Allow to override build user and host

[bmwiedemann]

Allow to override build user and host, using the standard $USER and $HOSTNAME environment variables to make reproducible builds possible.
See https://reproducible-builds.org/ for why this is good.

This helps improve issue #3759

[ompiteam-bot]

Can one of the admins verify this patch?

[jsquyres]

ok to test

[jsquyres]

Thanks for bringing this to our attention again; I admit it kinda fell off our radar. 😦

Are you looking for both this PR and #3779 to be merged? (it looks like that one kinda fell off the radar, too...)

Also, per the discussion in #3759, are you interested in an output line in ompi_info, something like "Reproducible build: yes|no"? (ompi_info is the tool users use to see information about the Open MPI installation)

[bmwiedemann]

I'd say, this change perfectly complements #3779

We could use $SOURCE_DATE_EPOCH as an indicator for "Reproducible build: yes"
but I'd prefer if we could keep the code simple and use this patch as is
and to compensate, in our .spec file we do something like

export HOSTNAME=reproducible-SUSE-rpm-build

which should show up in ompi_info as well

[jsquyres]

I'm good with this PR. Mind if I add a commit to it with a short README blurb about setting$USER and/or $HOSTNAME and citing Reproducible Builds? (what's the best URL to cite for RB's, anyway -- https://reproducible-builds.org/ ?)

[jsquyres]

@bmwiedemann Ping.

[bmwiedemann]

oops, missed the other comment. Yes, please add blurb with that URL.

[jsquyres]

@bmwiedemann I rebased to the current HEAD (since this commit was super old) and added a 3rd commit with a README blurb. Look ok?

@ggouaillardet You might want to expand on this README blurb in #3779...?

[bmwiedemann]

LGTM

[bmwiedemann]

Can you merge this PR, please?

[jsquyres]

Thanks for your patience -- merged!