[tlbuild] dvisvgm-2.1 trial build

[Karl Berry]

Hello TL builders - I've imported a major new version of dvisvgm (2.1)
into TeX Live. If you're up for it, feel free to try a build and see how
it fails :).

I believe that C++ as of 2011 is now required. The configure script checks
for this and uses, e.g., g++ -std=c++11, when that is supported.

I removed the various libgs configure-time options which peb had put in
at my request and just force dynamic loading (dlopen etc.). It's the
only thing that makes sense for native TL binaries as far as I can see.
If I'm wrong and you need some option or another back, let me know.

Akira has been making yeoman efforts keeping the TL source tree up to
date with releases of various libraries and programs, so there are
plenty of other changes to cause breakage as well :).


As long as I'm here: overall, my general idea is to start doing trial
builds in March, build a first test image in April, finish things up in
May. (http://tug.org/texlive has the usual set of not-set-in-stone dates.)

Thanks,
Karl