Changes: - 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.
Changes:
- 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
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