Enable the -Wundef warning for clang

webrtc/tools/simple_command_line_parser_unittest.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/tools/simple_command_line_parser.h"
 
 namespace webrtc {
 namespace test {
 
 class CommandLineParserTest : public ::testing::Test {
  protected:
   void SetUp() override {
     parser_ = new CommandLineParser();
 
@@ skipping 55 matching lines @@
 }
 
 TEST_F(CommandLineParserTest, GetCommandLineFlagValue) {
   EXPECT_EQ("", parser_->GetCommandLineFlagValue("--foo"));
   EXPECT_EQ("", parser_->GetCommandLineFlagValue("--foo-foo"));
   EXPECT_EQ("1", parser_->GetCommandLineFlagValue("--bar=1"));
 }
 
 }  // namespace test
 }  // namespace webrtc