Adding build_opus as a switch in GYP.

This is to allow not building Opus. On non-chromium non-gyp chases, one can let WebRTC depend on other Opus builds.

BUG=
R=kjellander@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43739004

Cr-Original-Commit-Position: refs/heads/master@{#8754}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7c112f3e5ae8b7d807f006c191291e5d4b56a38d
4 files changed