latex2svg: work around dvisvgm < 3.0 not detecting ghostscript 10.

latex2svg: work around dvisvgm < 3.0 not detecting ghostscript 10.

The latex stack and everything around it is ... interesting. A huge pile
of stuff that randomly breaks in very silent and non-obvious ways, and
it's all so huge and complicated that even distro package maintainers
have no way to know if the packages are mutually compatible.

Now that I figured this out, I still need to comment on that ArchLinux
bug.