I've filed https://github.com/mesonbuild/meson/issues/5462 about the
fact that vs_module_defs cannot accept an empty list (the usual way in
meson to say "nothing" (like NULL, None, etc).
This allows msvc to compile a bz2-1.dll that at least passes the tests
provided.
The .def file doesn't work for mingw due to (I think) differences in the
way msvc and mingw define .def files. I *think*, but I'm not positive,
it has to do with stdcall vs cdecl, but I'm just throwing stuff at the
wall till it sticks.
