- Add Clang as dependency to make building on Mac possible.
- Add gold linker for Unix platforms
- Replace Cygwin with WebRTC's slimmed down version for Win.

Dependencies updated to the Chrome revision WebRTC uses:
- GYP
- gtest
- libjpeg_turbo
- yasm and patched-yasm

TEST=Compiled and executed Debug+Release builds of libyuv_unittest on Mac, Linux and Windows.
Review URL: https://webrtc-codereview.appspot.com/758006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@327 16f28f9a-4ce2-e073-06de-1de4eb20be90