Fix clang warnings in GN build.

third_party/opus/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/arm.gni")
 
 # If fixed point implementation shall be used (otherwise float).
 use_opus_fixed_point = ((is_android || is_chromeos ||
                          (is_ios && arm_version == 7)) && cpu_arch == "arm")
 
@@ skipping 139 matching lines @@
     "src/celt",
     "src/silk",
   ]
 
   if (is_win) {
     defines = [ "inline=__inline" ]
   }
   if (is_android) {
     libs = [ "log" ]
   }
+  if (is_clang) {
+    cflags = [ "-Wno-absolute-value" ]
+  }
 
   deps = [ ":opus" ]
 }