src/Makefile: trying to integrate it with opam

I needed the union feature of macaque, present in the HEAD of its master branch,
but not in the version available in opam (http://opam.ocaml.org/packages/macaque/macaque.0.6.1/).
Therefore, I tried
% opam pin macaque https://github.com/ocsigen/macaque.git
but the build failed.

After my tests, this pull_request that implements quite an ugly build workaround,
opam pin macaque <http_url_to_this_pull_request>
seems to install a working version of macaque with the union feature.

I propose the PR in case other opam users don't have the time to wait for a cleaner version.