Use cmake to find a lua executable

A lua executable is now required for the build process since a lpeg-based script
is used for generating a dispatch function and metadata for the msgpack API
frontend. This removes the need for setting the LUA_BINARY environment variable.
Fixes #518.
Add checks for lpeg/cmsgpack lua packages

These checks will stop the build process with meaningful error messages if
any of those packages are not installed.
Add lpeg as explicit third-party dependency

It was being installed because of moonscript, but now it's listed as a
dependency explicitly.