[ios] - bump gas-preprocessor
[ios] - support Xcode 6.x and iOS 8 SDK

- use clang as default compiler for newer SDKs
- fakeroot: ignore interposing openat()
[ios] - fix libssh build with Xcode 6
[ios] - fix pil build with Xcode 6
[ios] - libRTV fix build for Xcode 6
ios] - fix libmpeg2 build error by making it even more PIC (also the real fix for the xcode5 build issue earlier)
[ios/atv2] - adapt readme - we support Xcode 6.0.1 against iOS 8 SDK
[osx] - adapt readme for compatibility with Xcode 6.1 with OSX SDK 10.10 under Yosemite
[ios/atv2] - adapt the readme for support with Xcode 6.1 against iOS SDK 8.1 on Yosemite
Merge pull request #5403 from Memphiz/xcode6

[ios] - add support for compiling with Xcode 6 and iOS 8 SDK