Recent iOS changes reverted

I believe https://github.com/alexcrichton/cmake-rs/pull/93 ended up causing https://github.com/alexcrichton/cmake-rs/issues/95 by accident so I've [reverted that change](https://github.com/alexcrichton/cmake-rs/commit/9a53f43ed9d338389950655738a9360cbfe27d01) while this is sorted through.

@simlay  would you be able to look into this?