Add compilation flag for building against LuaJIT or Lua VMs.

Replaces #528 with a direct merge into master.

`make` will continue to target the Lua VM by default. This adds a `make` option to generate a runtime build targeting LuaJIT. This isn't intended to be long-term (the Lua VM will just be removed eventually) but to provide greater opportunities for testing coverage before enabling it by default.

```
make ENABLE_LUAJIT=1
```