[tlbuild] --enable-hitex trial

[Karl Berry]

Hello TL builders - tl,dr: please try a test build of current TL with
--enable-hitex.  For now it is not enabled by default, but we hope to
include it in TL22.

That is: ./Build --enable-hitex <other args>; or passing the arg to
configure, if you do your own configure && make.

We've only compiled on our own systems (Slackware, CentOS, Windows),
so there are surely portability problems. Hence this msg/request.

What it is: As some of you may know from his various articles and talks
and svn commits, Martin Ruckert has been developing a new engine "hitex".
  https://hint.userweb.mwn.de/
  https://tug.org/TUGboat/Contents/listauthor.html#Ruckert,Martin

This is that engine.  It uses his HINT output format, intended for
display on mobile devices.  There are auxiliary programs histretch and
hishrink to make hint files human-readable / compress-to-binary, a la
tftopl / pltotf, etc.  These are in TL too, and compiled when hitex
itself is.

Martin currently has HINT viewers for Android and Windows, plus one in
development for (GNU/)Linux, linked from his web page. They rely on
OpenGL. We may add the Linux viewer to TL as a separate "extra" a la
asymptote, but let's get the engine working first.

Thanks,
Karl