Enable the -Wundef warning for clang

webrtc/modules/audio_processing/beamformer/covariance_matrix_generator.cc

 /*
  *  Copyright (c) 2014 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.
  */
 
 #define _USE_MATH_DEFINES
 
 #include "webrtc/modules/audio_processing/beamformer/covariance_matrix_generator.h"
 
 #include <cmath>
 
 namespace webrtc {
 namespace {
 
 float BesselJ0(float x) {
-#if WEBRTC_WIN
+#ifdef WEBRTC_WIN
   return _j0(x);
 #else
   return j0(x);
 #endif
 }
 
 // Calculates the Euclidean norm for a row vector.
 float Norm(const ComplexMatrix<float>& x) {
   RTC_CHECK_EQ(1u, x.num_rows());
   const size_t length = x.num_columns();
@@ skipping 73 matching lines @@
     float distance = std::cos(angle) * geometry[c_ix].x() +
                      std::sin(angle) * geometry[c_ix].y();
     float phase_shift = -2.f * M_PI * distance * freq_in_hertz / sound_speed;
 
     // Euler's formula for mat[0][c_ix] = e^(j * phase_shift).
     mat_els[0][c_ix] = complex<float>(cos(phase_shift), sin(phase_shift));
   }
 }
 
 }  // namespace webrtc