[WIP] use Dune

Changes:

• Makefile.config is not installed anymore
• four dune sites:
  • (why3 datadir): for adding to stdlib and drivers
  • (why3 libdir): for adding .vo
  • (why3 plugins): for new plugins
  • (why3 cmds): for new commands why3 mycmd

For testing:

• opam pin "git+ssh://git@github.com/ocaml/dune.git#master"
• opam pin camlzip --kind=git "https://github.com/xavierleroy/camlzip#f5aba998b6b76c5b73da78047fad166a645f1593"

todo:

• doc for the dune sites
• cleanup the Makefile.in (done late in order to avoid too many conflict)
• add why3.infer
• test relocation dune install --relocation --prefix $DIR