[tlbuild] tl17 trial builds

[Karl Berry]

    only available with -std=c++11

Ok, that's expected. We'd have to change our configure to insert that
flag, as the original poppler configure did.

But since Akira has reverted poppler to the previous version in the
meantime, I guess we will go that route for this release. So you can go
back to Debian 6 if you want.

    configure: error: *** A compiler with support for C++11 language features is required.

It is sad that even the compiler on Debian 7 is not good enough for
dvisvgm. The configure macro doesn't just check for the presence/absence
of the flag, but checks a lot of actual language features:
  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html

Next year there may be no choice but to require C++11, if this trend
continues ... -k