Simplified and optimzed statement

Ignore-this: 24d9b63f0ff54079d346c6dbdb94e7d8

darcs-hash:20131206021418-b22ec-82429776baf9dae6f10fc13107aab6dcae6d500e
Don't compile WebRTC AEC if machine is not x86 or x86_64

Ignore-this: a04deb12845543198da7447674e72da5

darcs-hash:20131210095148-56b4d-76853e87918c9bb44ba2e806f71b381e61829c60