Enable the -Wundef warning for clang

NOPRESUBMIT=true
BUG=webrtc:6398

Review-Url: https://codereview.webrtc.org/2358993004
Cr-Original-Commit-Position: refs/heads/master@{#14425}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 77eab704707e03c74eda3126b92e4ef5e2b7eb4d
515 files changed