SOURCE_DATE_EPOCH support

[Karl Berry]

Hi Werner,

    do Metafont and Metapost support the `SOURCE_DATE_EPOCH` environment
    variable?

From
 grep -anr source_date_epoch # in Build/source/texk/web2c/
it looks like only pdftex and luatex.

If you want to request support in mpost, I suggest writing
mp-implementors at tug.org.

As for Metafont, if you or anyone can make a patch (I doubt it is deep),
I'll install it, but I can't commit to looking into it myself for quite
a while. --best, karl.