Make WARN_UNUSED_RESULT a no-op on gcc

Because on gcc, cast to void doesn't silence the warning. See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

Also add an RTC_ prefix to the macro instead of only defining it if it
wasn't already defined, to ensure that we always get our own version.

BUG=none

Review-Url: https://codereview.webrtc.org/2797983003
Cr-Commit-Position: refs/heads/master@{#17563}

webrtc/modules/audio_coding/codecs/ilbc/cb_construct.h

@@ -34,6 +34,6 @@ bool WebRtcIlbcfix_CbConstruct(
     int16_t* mem,              /* (i) Buffer for codevector construction */
     size_t lMem,               /* (i) Length of buffer */
     size_t veclen              /* (i) Length of vector */
-    ) WARN_UNUSED_RESULT;
+    ) RTC_WARN_UNUSED_RESULT;
 
 #endif