Integrate Coq's MenhirLib
This addresses #1 by adding the corresponding option. Moreover, Coq's MenhirLib is now part of the default distribution.
opam file contains the necessary tricks to make sure that MenhirLib is only installed when Coq is.
I just figured out that if these changes suit you, we will need to make some adjustments to the license. But nothing really worrying, since Menhir support libraries in Coq are already distributed under the GPL (without linking exceptions) as part of CompCert.