[tlbuild] dvisvgm-2.1 trial build

[Karl Berry]

I knew C++11 would be a big barrier.  Martin (Gieseking) wrote me:

    Yes [...] As of dvisvgm 2.0, C++11 is required. The new language
    features and the additions to the standard library allow much
    cleaner code with less cumbersome constructions.  The sources should
    compile fine with gcc 4.8.1, clang 3.3, or any later versions.

Since I doubt it is feasible or desirable to build the entire source
with compilers of that vintage (even when they are available), as far as
I can see the choices are:

1) --disable-dvisvgm
2) build dvisvgm in a separate tree from everything else with different
options.

I expect the result will be that dvisvgm is no longer included in the
native TL on most platforms, and that's ok (since it has to be).

Best,
Karl