Allow to override man-page date

[bmwiedemann]

to make package build 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.

[rrthomas]

Since this repo is now unmaintained, it would be great if you could resubmit this pull request, suitably rebased, to rrthomas/recode. I'm a fan of reproducible builds, so would be happy to help land it!