optionnal drop posix compat and allow to handle long option name with one dash

I would really like to use cmdliner for the ocsigen stack.
But I need cmdliner to drop POSIX compat for the following reason: 
- `eliomc` is a (light) wrapper arround `js_of_ocaml`/`ocamlc`/`ocamlfind` and pass some of its command line arguments directly to the subprocess. 
- ocamlfind and the ocaml compiler doesn't follow POSIX's `-oVALUE` convention.

Possible solutions:
  - the PR get accepted :)
  - fork :/
  - keep posix and replace double dash ('--' -> '-') before passing argument to `ocamlfind`/`ocamlc`. :'(

One issue remains ! How to handle `ocamlc`/`js_of_ocaml` arguments inside `eliomc`.
  - How to display help msg
  - How to detect unknown arguments ?