Support for XCode 6

@jlipps I am unsure what version of XCode the files in `thirdparty/iwd`. Running on XCode 6, the previous code would fallback to using those files and Instruments would still end up running with a delay in the simulator. I complied the latest version of [Instruments Without Delay](https://github.com/facebook/instruments-without-delay) (which supports XCode 6) and tried using those files instead; those work just fine.