[ocaml-infra] opam installext packaged up

[Anil Madhavapeddy]

Hi,

I've been cleaning up some of the bulk building infrastructure, and so have packaged up opam-installext into an OPAM package as Thomas Gazagnaire suggested a while back:

https://github.com/ocaml/opam-repository/pull/3296

Testing and support for more OS distributions would be much appreciated.  When the pull request is merged, you just need to:

    opam update
    opam install opam-installext
    opam installext ssl

The configure script is pretty straightforward to edit in the source:

    https://github.com/avsm/opam-installext

If there are no objections, I will transfer this repository to the `ocaml` organisation on GitHub soon for a permanent home.

cheers,
Anil