track dependencies on files installed by ocamlfind packages

[samoht]

It would be nice if ocamlbuild main.native would recompile my program if one of the ocamlfind dependencies changed underneath (because of an opam reinstall for instance). @diml wrote some rules to do that there. Would be great if it was the default in ocamlfind -use-ocamlfind, though.

[gasche]

It's a good idea; it may need a bit more work to be portable, though, for example assuming uname, md5sum, stats and xargs available in the shell is a bit much for something that could be done reasonably within the Unix module and would then have at least a chance of working on Windows.