[tlbuild] --disable-dvisvgm avoids configure

[Karl Berry]

I've tweaked the TL build system so that --disable-PKG (e.g.,
--disable-dvisvgm) avoids descending into the PKG/ (dvisvgm/) directory
entirely, so configure is never run. As a consequence, it will not abort
when a C++11 compiler is not found :).

So, for those of us without C++11,
./Build --disable-dvisvgm should now suffice.

I've tried a few builds and they worked out, but it won't surprise me in
the slightest if this change has introduced newly strange behavior of
some kind. Let me know if problems ... --thanks, karl.