Enable the -Wundef warning for clang

webrtc/base/ignore_wundef.h

+/*
+ *  Copyright (c) 2016 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_BASE_IGNORE_WUNDEF_H_
+#define WEBRTC_BASE_IGNORE_WUNDEF_H_
+
+// If a header file uses #if on possibly undefined macros (and it's for some
+// reason not possible to just fix the header file), include it like this:
+//
+//   RTC_PUSH_IGNORING_WUNDEF()
+//   #include "misbehaving_header.h"
+//   RTC_POP_IGNORING_WUNDEF()
+//
+// This will cause the compiler to not emit -Wundef warnings for that file.
+
+#ifdef __clang__
+#define RTC_PUSH_IGNORING_WUNDEF() \
+  _Pragma("clang diagnostic push") \
+      _Pragma("clang diagnostic ignored \"-Wundef\"")
+#define RTC_POP_IGNORING_WUNDEF() _Pragma("clang diagnostic pop")
+#else
+#define RTC_PUSH_IGNORING_WUNDEF()
+#define RTC_POP_IGNORING_WUNDEF()
+#endif  // __clang__
+
+#endif  // WEBRTC_BASE_IGNORE_WUNDEF_H_