consensus lib work: some win build fixes necessary for dll creation

This is part of the build-system work needed for the upcoming consensus lib.

- Cut down on the dll's linked globally to avoid a few mingw bugs (though it's a legitimate cleanup anyway).
- Split out LIBS for Windows builds as was done for INCLUDES in 6b099402b40dcf9fc716be29c85e7e1865b28e92.

This should result in no noticeable changes.