Overhaul build using luarocks

The upstream source release tarballs do not include a rockspec (or licensce or other goodies) and hence the atempts to manually install the bits ends up with no metadata to build manifest from. This means luarocks list will not find this even though it is installed. Since upstream actually recommends luarocks as the "easy way" anyway, lets just do that...

For an example of why this can be an issue see discussion here: https://github.com/sile-typesetter/sile/issues/2118