vstte'10 competition : problem 3 in progress (but there seems to be a bug in Why which prevents contructor None to be visible)

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Multi-target is required for ocamlyacc but is counterproductive for ocamlc.
This is due to ocamldep which declare .cmi as dependencies only when
a .mli file exists.

That work one day because %.cmi was wrongly spelled %cmi, after the
spelling correction that doesn't work anymore. Until now :)

The last commit is also an improvement for -j

en utilisant parser.pre.mly (le seul fichier à modifier)