[tlbuild] tl17 trial builds

[Karl Berry]

Hello TL builders - Akira and I have updated most of the version numbers
in the source tree for TL17, and Akira has done yeoman efforts keeping
third-party libraries and programs up to date.

So now would be a good time to try a trial build to make sure there are
no (more) unwelcome surprises.

Don't commit any new binaries yet. We're not nearly ready for that; I
expect to start the actual pretest in mid-April. For now, stick to
messing around in the source/ tree only.

Asymptote remains un-updated. Maybe in another day or two we'll have a
non-C++11 version to try.

There will be even more changes to come in LuaTeX (I don't have an ETA),
but it is already substantially updated from TL16; it's now version 1.0.4.

There's a chance that we will have new versions of clisp and its
hairy/harrowing dependencies, for xindy purposes, but we'll have to see
if and when that is available. For now, no changes in xindy/clisp.

dvisvgm will remain with its C++11 requirement. For platforms that
cannot compile it, Martin has concurred that we may as well continue to
distribute the binaries from TL16 (1.15.1), as the main functionality is
unchanged. I'll arrange for that in the tl-update-bindir script when the
time comes.

I think that's the status. Off to work on the doc drafts now ... -k