- New fonction in plugins: main_loop_body_prefix

Impact:
- EMF backend has an extra machines argument
- specific option to avoid merge of ite constructs
- set_backend function to improve backend selection

Most code was extracted from seahorn_backend through c0f8

a6df3 is the initial commit of branch seahorn

- allow multiple -I dir, will be used in order (first one declared
  is first used)

- when declaring a global library #open <foo>, foo is first checked
  in local folders, than in global one (install path). This
  does not apply to local libraries opened with #open "foo".

- Version now includes current git branch

- no more classic display for ocamlc
- compilation warnings removed

Non regression results were similar to master branch