feat: Allow external build scripts to link Lua libraries

[Sculas]

This PR makes a small change to mlua-sys (and mlua to propagate the feature) that allows external crates and build scripts to do the compilation and linking part of the Lua libraries.

For my use case, I'm working on Pluto bindings for mlua (which I'll release very soon). Pluto allows for many sandboxing options, which is why I created a separate build script for it. In this script, you can configure each of those options to suit your needs.

In this case, the Pluto build script also handles the compilation and linking of the static library. This PR allows this system to work.
This is also useful for any future Lua forks/derivatives since you're no longer bound to what mlua can compile, instead you just need to confirm that the ABIs match.