For those who would want to use bitstring on mingw, I had it compiled ok 
with the following:
> cp byteswap.in.h byteswap.h
> sed -i -e 's/-Werror -fPIC//' Makefile.in
> sed -i -e 's/open_in/open_in_bin/' pa_bitstring.ml 
> create_test_pattern.ml bitstring_objinfo.ml
> ./configure CFLAGS="-mno-cygwin"
> make


The test target is also buggy on mingw.

But the library is working ok

Salutations

Matt