Enable the -Wundef warning for clang

webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm

 /*
  *  Copyright (c) 2013 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 29 matching lines @@
   NSCondition* _captureChangingCondition;
 }
 
 @synthesize frameRotation = _framRotation;
 
 - (id)initWithOwner:(VideoCaptureIos*)owner captureId:(int)captureId {
   if (self == [super init]) {
     _owner = owner;
     _captureId = captureId;
     _captureSession = [[AVCaptureSession alloc] init];
-#if defined(__IPHONE_7_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_7_0
+#if defined(__IPHONE_7_0) && defined(__IPHONE_OS_VERSION_MAX_ALLOWED) \
+    && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_7_0
     NSString* version = [[UIDevice currentDevice] systemVersion];
     if ([version integerValue] >= 7) {
       _captureSession.usesApplicationAudioSession = NO;
     }
 #endif
     _captureChanging = NO;
     _captureChangingCondition = [[NSCondition alloc] init];
 
     if (!_captureSession || !_captureChangingCondition) {
       return nil;
@@ skipping 318 matching lines @@
 }
 
 - (void)waitForCaptureChangeToFinish {
   [_captureChangingCondition lock];
   while (_captureChanging) {
     [_captureChangingCondition wait];
   }
   [_captureChangingCondition unlock];
 }
 @end