[tlbuild] pmx compilation

[Karl Berry]

FYI, Peter and Bob (thanks!) have integrated pmx into the TL build as
was discussed previously.  (Cross-compilation doesn't work, that'll have
to come later.)

Might be worth giving it a shot now to see if there are problems.
--disable-pmx to configure will omit it.

There are MacOSX and Windows binaries on CTAN that would include if
compilation fails on those platforms.

Best,
Karl