[tlbuild] building for distribution with shared breaks in dvisvgm

[Karl Berry]

    np> The file is there in dvisvgm-src/libs/xxHash/xxhash.h but that
    path is not added to the include path like clipper etc.

Norbert: Without --enable-bundled-libs, my surprise is more that clipper
is included than xxhash is not. But I don't know Martin's intent for the
different libraries.

Martin: please see https://tug.org/pipermail/tlbuild/2018q3/004267.html
for Norbert's original report. --thanks, karl.