If SOURCE_DATE_EPOCH is defined, use it for copyright year

If SOURCE_DATE_EPOCH is defined, use it for copyright year

Using the date from SOURCE_DATE_EPOCH instead of the current date makes
it possible to reproduce a build that was built on a different year:
https://reproducible-builds.org/specs/source-date-epoch/

This is fixing an issue we had while building Tor Browser:
https://trac.torproject.org/projects/tor/ticket/33535

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/11296)