Towards a bridge with ocamlfind

This series of patches completely removes the support for custom compilation options which were in OPAM since its beginning. This feature was complex and useless, and I'm sure nobody will miss it. Now we have a clean and clear semantics: every package can defined a `<pkg>.config` file at this root, with global variables which can be read by other packages if necessary.

We can now use the `library` and `syntax` field which were here from the beginning, and start filling them with interesting findlib metadata. I've push an example of automatic extraction to opam-repository soon.