Annotate COMPILE_ASSERT with __attribute__((unused)).

Also renames UNUSED -> ATTRIBUTE_UNUSED to be able to use this when
building peerconnection_jni.cc which apparently has this defined to
something else.

R=kjellander@webrtc.org
TBR=mflodman@webrtc.org
BUG=4018

Review URL: https://webrtc-codereview.appspot.com/28039005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7711 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_processing/aec/aec_core.c

@@ -1356,7 +1356,7 @@ static void ReopenWav(rtc_WavWriter** wav_file,
                       int seq1,
                       int seq2,
                       int sample_rate) {
-  int written UNUSED;
+  int written ATTRIBUTE_UNUSED;
   char filename[64];
   if (*wav_file) {
     if (rtc_WavSampleRate(*wav_file) == sample_rate)