build: embed reproducible timestamp in manpages
build: embed reproducible timestamp in manpages

If the environment variable SOURCE_DATE_EPOCH is set in the build
environment, prefer it over the current date when determining the
correct timestamp to use as the date the manpages were built.

See https://reproducible-builds.org/specs/source-date-epoch/

github/hub,github/hub,github/hub,github/hub