Make lib/dns/gen.c compatible with reproducible builds.

The gen.c will now use SOURCE_DATE_EPOCH[1] if found in environment
to make the build more reproducible build friendly.

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

(cherry picked from commit c8cb612d)
42 jobs for !1913 with ondrej/reproducible-build-v9_11 in 18 minutes and 40 seconds (queued for 4 seconds)
merge request