Update BER MetaOCaml to run on native Windows

[dra27]

Repo at https://github.com/metaocaml/ber-metaocaml

It all basically works:

• On mingw-w64, update ber-metaocaml-114/metaocamlc.c to propagate the exit code of the underlying tool correctly (execve is implemented on Windows, but has the wrong semantics)
• On MSVC, update ber-metaocaml-114/metaocamlc.c to remove POSIX-isms, starting with the use of unistd.h
• Update the testsuite to cope with CRLF output (that may involves using different diff options, stripping the CRs before comparison or going the whole hog and using ocamltest's lovingly crafted line-ending-agnostic file comparison function)