Makefile.inc1: Make package timestamps reproducible by default

Set package archive timestamps based on most recent source commit
timestamp (approach suggested by bapt).

I'd like to include git metadata in a file included in src tarballs, so
that the build is reproducible (including the hash shown in uname etc.)
outside of a git checkout. There are still details to be sorted out to
do that, so this is an interim step to improve reproducibility.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49165