Preliminary support for dune

- dune build matita/matita{,c,clean}.exe works
- dune build fails on components/syntax_extensions where we need
  to add modules to two different libraries. I have found a
  workaround (dune does not allow it!) that works when building
  matita*.exe, but not when doing "dune build"
- the dune files and the Makefiles are not 100% equivalent
  (e.g. some modules not dropped during compilation, the user
  interface not generated by glade yet)