a patch to Makefile.menhir for ocaml-non-native architctures.

[yozot]

OpenBSD ports recently updated CompCert to 3.13, but does not work for ocaml-non-native architectures. The problem comes from Makefile.menhir expecting either menhirLib.cmxa or .cmx is available. It assumes ocaml native compilation environment.

This patch adds one more condition using OCAML_NATIVE_COMP which is prepared by configure, to adjust MENHIR_LIBS according to ocaml-native or ocaml-non-native environment. I confirmed this patch works in OpenBSD ports, and believe it's useful for any other environments.