[tlbuild] dvisvgm fix for non-native build

[Karl Berry]

    +if test "x$have_libgs" = xyes; then
    +  HAVE_LIBGS=1
    +fi

I don't totally remember, but I think I may have taken that out
precisely because we do not ever want to link with -lgs for TL. I guess
it could be done with an additional test for a TL build.

    Subject: [tlbuild] dvisvgm fix for non-native build

In general, my recommendation is to build Martin's original dvisvgm
release if you're not building for TL. He has a good setup, but the TL
configure cannot easily be maintained with both linking with
system-everything the way he does it, vs. TL-everything the way we need
to. --thanks, karl.