rebar_agner_compiler can be implemented that will invoke necessary build steps for non-rebar compatible projects. The only thing is that we need agner to be able to tell "what this repo is" — while possible with gt/hg/svn/bzr, going to be a little bit tougher with tarballs. Although with tarballs what we can do is to put some file into every checked out tarball referring to its package
In fact, 149e6dd stores the whole spec in fetched directory, and this is definitely something to use in rebar.
Most likely to be superseded by agner/agner#131
So it was.