Add a gyp variable to fully disable Opus.

Without this, the Chromium gyp run will fail until Opus is added.

This builds and runs Opus on Linux with include_opus=1, and builds with include_opus=0 (including with third_party/opus deleted to simulate the Chromium build).

Review URL: https://webrtc-codereview.appspot.com/902005

git-svn-id: http://webrtc.googlecode.com/svn/trunk/src@2953 4adac7df-926f-26a2-2b94-8c16560cd09d