Enable the -Wundef warning for clang

webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm

Index: webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm
diff --git a/webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm b/webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm
index a2af4afc0c4eddbf9e51ac2cdf3378ff2e07820a..10671eb2aee6b0657688c7b4b021967262901e60 100644
--- a/webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm
+++ b/webrtc/modules/video_capture/objc/rtc_video_capture_objc.mm
@@ -47,7 +47,8 @@ using namespace webrtc::videocapturemodule;
     _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;