Port build system to VimL function

[alerque]

Having GNU-Make as a dependency to install is not ideal. It is easy on Linux but might be a hinderance on other platforms. Really nothing much special needs to happen beyond a cargo command and some file movement. This could be ported to a VimL function (or Lua, since Lua support is a requirement already) to ease adoption.

The biggest issue will probably be detecting and using the right Lua headers when buliding the Lua module. Currently the Makefile only assumes the Arch Linux path anyway. Actually pkg-config could supply this on most systems, but the mlua Rust crate won't build against Arch's version of LuaJIT unless you force it by setting the variables manually (see mlua-rs/mlua#3).