Enable the -Wundef warning for clang

webrtc/modules/audio_device/ios/audio_device_ios.mm

 /*
  *  Copyright (c) 2012 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.
  */
 
@@ skipping 66 matching lines @@
   @autoreleasepool {
     LOG(LS_INFO) << " system name: " << ios::GetSystemName();
     LOG(LS_INFO) << " system version 1(2): " << ios::GetSystemVersionAsString();
     LOG(LS_INFO) << " system version 2(2): " << ios::GetSystemVersion();
     LOG(LS_INFO) << " device type: " << ios::GetDeviceType();
     LOG(LS_INFO) << " device name: " << ios::GetDeviceName();
     LOG(LS_INFO) << " process name: " << ios::GetProcessName();
     LOG(LS_INFO) << " process ID: " << ios::GetProcessID();
     LOG(LS_INFO) << " OS version: " << ios::GetOSVersionString();
     LOG(LS_INFO) << " processing cores: " << ios::GetProcessorCount();
-#if defined(__IPHONE_9_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_9_0
+#if defined(__IPHONE_9_0) && defined(__IPHONE_OS_VERSION_MAX_ALLOWED) \
+    && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_9_0
     LOG(LS_INFO) << " low power mode: " << ios::GetLowPowerModeEnabled();
 #endif
   }
 }
 #endif  // !defined(NDEBUG)
 
 AudioDeviceIOS::AudioDeviceIOS()
     : audio_device_buffer_(nullptr),
       audio_unit_(nullptr),
       recording_(0),
@@ skipping 735 matching lines @@
 
   // All I/O should be stopped or paused prior to deactivating the audio
   // session, hence we deactivate as last action.
   [session lockForConfiguration];
   UnconfigureAudioSession();
   [session endWebRTCSession:nil];
   [session unlockForConfiguration];
 }
 
 }  // namespace webrtc