Add an opam file

Because of autoconf, the dev repository is not pinnable in opam. This opam file (which is directly copied from the opam repository) fixes this.
If you want to use the *same* file whether in the repo or in the opam repository you could add a filter here 

```["autoconf"] { %{ocamlgraph:pinned}% }```

(Not exactly sure about the syntax though).