[tlbuild] building for distribution with shared breaks in dvisvgm

[Karl Berry]

            Build/source/texk/dvisvgm/configure.ac
    to carry over the stuff from
            Build/source/texk/dvisvgm/dvisvgm-source/configure.ac

It's not just configure.ac, but the Makefile.am's. Dvisvgm's wide use of
other libraries means a system build of dvisvgm is harder to support in
TL than other programs, and I did not see a good way to support both. At
least without a whole lot of work with conditionals, etc. If you want to
put in the work, and you are willing to maintain them in perpetuity or
Martin is willing to accept the changes, go right ahead :). I feel like
I've put in my time struggling with this, and Martin has put in his time
making the native TL build easier in the first place.

Meanwhile, if/since you want to build dvisvgm standalone, my advice is
to --disable-dvisvgm when doing the TL build and download/install the
program separately from the original source in Debian. -k