Make check_subsystem_order work in distcheck builds.

        return open(fname)

def check_subsys_file(fname, uses_dirs):

    if not uses_dirs:

        # We're doing a distcheck build, or for some other reason there are

        # no .may_include files.

        return False

    uses_dirs = { normalize_srcdir(k) : { normalize_srcdir(d) for d in v }

                  for (k,v) in uses_dirs.items() }

    uses_closure = closure(uses_dirs)