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.

Without this patch, the epics-base package varied in html docs
for every build.

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

[khwilliamson]

Karen, this looks good to me, but I would like a more expert opinion.

[karenetheridge]

looks good, but can we name the environment variable PERL_POD_SIMPLE_SOURCE_DATE_EPOCH to make it abundantly clear who the consumer of this variable is intended to be?

[bmwiedemann]

Renaming the variable would be bad, because SOURCE_DATE_EPOCH is a standard name already consumed by dozens of other tools and to make this reproducible then would need packagers to discover and use this special variable.

[karenetheridge]

SOURCE_DATE_EPOCH is a standard name already consumed by dozens of other tools

Ah I see; good to know!

[khwilliamson]

How soon do you need a release

[bmwiedemann]

It is not urgent. reproducible-builds is a long-term project, so if this ends up in perl in 2021, that will be fine.