[tex-k] Reproducible builds using pdftex

[Karl Berry]

    SOURCE_DATE_EPOCH is used to define the date/epoch and is evaluated
            for pdf output etc
    SOURCE_DATE_EPOCH_FIXUP_TEX_PRIMITIVES = 0|1
            if non-0 then also \today is fixed.

Agreed.  That's what I meant to propose yesterday, but confused myself
as I was writing.

Thanks for the draft patch.  I'll look at it.  It will take far more
time to test and document than anything else :(.  -k