Describe the various Cmdstan and Stan makefile variables

[rok-cesnovar]

Summary:

There are multiple makefile variables that are not document anywhere, mostly those that are mostly useful during development. For example STANC3, STANC3_VERSION.

We need to document them somewhere. I am not sure the CmdStan guide is the best place - I don't want to confuse users with makefile variables that are mostly useful in Stanc3/CmdStan development.

Current Version:

v2.29.2

[WardBrian]

I think repository wikis or in make/local.example (or both) would be a good place for this kind of information