Enable the -Wundef warning for clang

webrtc/base/gtest.h

Index: webrtc/base/gtest.h
diff --git a/webrtc/base/gtest.h b/webrtc/base/gtest.h
new file mode 100644
index 0000000000000000000000000000000000000000..24a46f8651c60c019eb4ac57a89ce23f9c4095ed
--- /dev/null
+++ b/webrtc/base/gtest.h
@@ -0,0 +1,27 @@
+/*
+ *  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_GTEST_H_
+#define WEBRTC_BASE_GTEST_H_
+
+#include "webrtc/base/ignore_wundef.h"
+
+RTC_PUSH_IGNORING_WUNDEF()
+#include "testing/gtest/include/gtest/gtest.h"
+RTC_POP_IGNORING_WUNDEF()
+
+// GTEST_HAS_DEATH_TEST is set to 1 when death tests are supported, but appears
+// to be left unset if they're not supported. Rather than depend on this, we
+// set it to 0 ourselves here.
+#ifndef GTEST_HAS_DEATH_TEST
+#define GTEST_HAS_DEATH_TEST 0
+#endif
+
+#endif  // WEBRTC_BASE_GTEST_H_