[tex-live] Extending SOURCE_DATE_EPOCH support

[Karl Berry]

    The patches for the other engines look definitely worth considering,
    and I consider sending it with explanations to the tex-k at tug.org

Patches for luatex should go to the dev-luatex list
(http://www.ntg.nl/mailman/listinfo/dev-luatex).

Patches for xetex are probably best submitted to their sf repository,
https://sourceforge.net/p/xetex.  The mailing list would be
http://lists.tug.org/xetex, afaik.

I don't think we should even consider making this change in luatex for
tl16 at this late date, since its source is independent and thus there
is great potential for problems.  xetex is slightly less problematic,
but ... we'll see.  --thanks, karl.