Fixes for compilation to iOS:

- remove /usr/include from the include path since the iOS SDK provides the correct version
- `_NSGetEnviron()` is private and not available on iOS
- `.align` without an argument is not allowed with the Apple tools. 2^2 should be the default alignment
- ignore error messages for XCode < 5
- pass include path to libuv
auto merge of #10227 : kud1ing/rust/ios, r=alexcrichton