Enable the -Wundef warning for clang

webrtc/modules/utility/source/helpers_ios.mm

 /*
  *  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.
  */
 
@@ skipping 160 matching lines @@
 std::string GetOSVersionString() {
   NSString* osVersion =
       [NSProcessInfo processInfo].operatingSystemVersionString;
   return StdStringFromNSString(osVersion);
 }
 
 int GetProcessorCount() {
   return [NSProcessInfo processInfo].processorCount;
 }
 
-#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
 bool GetLowPowerModeEnabled() {
   if (isOperatingSystemAtLeastVersion(9.0)) {
     // lowPoweredModeEnabled is only available on iOS9+.
     return [NSProcessInfo processInfo].lowPowerModeEnabled;
   }
   LOG(LS_WARNING) << "webrtc::ios::GetLowPowerModeEnabled() is not "
                      "supported. Requires at least iOS 9.0";
   return false;
 }
 #endif
 
 }  // namespace ios
 }  // namespace webrtc
 
 #endif  // defined(WEBRTC_IOS)