Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/h264/include/h264.h

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  *
  */
 
 #ifndef WEBRTC_MODULES_VIDEO_CODING_CODECS_H264_INCLUDE_H264_H_
 #define WEBRTC_MODULES_VIDEO_CODING_CODECS_H264_INCLUDE_H264_H_
 
 #if defined(WEBRTC_IOS) || defined(WEBRTC_MAC)
 
 #include <Availability.h>
-#if (defined(__IPHONE_8_0) &&                            \
-     __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_8_0) || \
+#if (defined(__IPHONE_8_0) && defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && \
+     __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_8_0) ||                  \
     (defined(__MAC_10_8) && __MAC_OS_X_VERSION_MAX_ALLOWED >= __MAC_10_8)
 #define WEBRTC_VIDEO_TOOLBOX_SUPPORTED 1
 #endif
 
 #endif  // defined(WEBRTC_IOS) || defined(WEBRTC_MAC)
 
 #include "webrtc/modules/video_coding/include/video_codec_interface.h"
 
 namespace webrtc {
 
@@ skipping 16 matching lines @@
  public:
   static H264Decoder* Create();
   static bool IsSupported();
 
   ~H264Decoder() override {}
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_CODECS_H264_INCLUDE_H264_H_