Added [Makefile.menhir], which gives a choice between Menhir's "code" and "table" back-ends when compiling CompCert.
For now, MENHIR_TABLE is set to false, so CompCert is not affected.
Setting MENHIR_TABLE to true builds CompCert using Menhir's table back-end.
This causes a small but repeatable slowdown on "make test", about 2% (roughly 1 second out of 40).
I have tested building ccomp and ccomp.byte.
I have tested with an ocamlfind-installed menhir and with a manually-installed menhir.