Change the ranlib detection

On Windows, the ranlib in the path may not be the right ranlib (it may
be the 32bit ranlib when we are making a Win64 compiler, or vice-versa).
Therefore we can't leave it up to libffi to detect the right ranlib, but
need to tell it which ranlib to use. This means that we need to find
ranlib even if we don't actually need it ourselves.