[tex4ht] %%~ expands to $TEXMFDIST

[Karl Berry]

A copy of a message I just sent to the tldistro list, for the record
here.

.. for TeX Live 2019,
the t4ht program will expand %%~ to $TEXMFDIST (set in texmf.cnf)
instead of $SELFAUTOPARENT. This means the tex4ht.env file should use
stuff like
  i%%~/tex4ht/ht-fonts/iso8859/1/!
instead of
  i%%~/texmf-dist/tex4ht/ht-fonts/iso8859/1/!

Perhaps distro versions have already been doing this, since
the SELFAUTOPARENT expansion was probably not useful, but I thought it
was worth mentioning.

References:
	https://tug.org/pipermail/tex4ht/2018q2/001975.html (thread)
	https://puszcza.gnu.org.ua/bugs/index.php?394

TL19, including this change, is currently being pretested --
https://tug.org/texlive/pretest.html. If anyone has patches to be
included, please let me know asap.

Thanks,
Karl