This commit adds support for building Snowflake (and its associated
components, webrtc and go-webrtc) with our updated toolchain that uses
clang 3.9.1 and macOS SDK 10.11

For clang, we add the '-Wno-unknown-warning-option' flag; see
https://bugs.llvm.org/show_bug.cgi?id=24979 for more information.