Add CMakeLists for embedded building

[connor horman <chorman64@...>]

Hello lua-l list,

I work a lot on projects that use lua, and build with CMake.

There are a few issues (in my opinion) that arises with this:

* The use of ExternalProject is required for lua, as it does not have a CMakeLists

* The build of lua is not relocatable, requiring either the sources be copied from the source dir to the binary dir, or an in-source build (which is not recommended by cmake)

The second in particular requires a hacky solution, as building in-source prevents reuse of the the source directory. Lua generally doesn't play nice with relocatable build systems (I originally noted this back in october, when I used autotools).

The implementation of a CMakeLists should be fairly trivial, I'm mainly wondering if something like that would be accepted before trying to figure out the makefile. 

Having an autotools configure would probably also be useful for people who still use that.