Revert of Remove the rtc_relative_path GYP variable and similar defines

webrtc/pc/externalhmac.cc

 /*
  *  Copyright 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.
  */
 
 #if defined(HAVE_SRTP) && defined(ENABLE_EXTERNAL_AUTH)
 
 #include "webrtc/pc/externalhmac.h"
 
 #include <stdlib.h>  // For malloc/free.
 
 extern "C" {
+#ifdef SRTP_RELATIVE_PATH
+#include "crypto_kernel.h"  // NOLINT
+#include "srtp.h"  // NOLINT
+#else
 #include "third_party/libsrtp/srtp/crypto/include/crypto_kernel.h"
 #include "third_party/libsrtp/srtp/include/srtp.h"
+#endif  // SRTP_RELATIVE_PATH
 }
 
 #include "webrtc/base/logging.h"
 
 // Begin test case 0 */
 static const uint8_t kExternalHmacTestCase0Key[20] = {
   0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
   0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
   0x0b, 0x0b, 0x0b, 0x0b
 };
@@ skipping 116 matching lines @@
       const_cast<auth_type_t*>(&external_hmac), EXTERNAL_HMAC_SHA1);
   if (status) {
     LOG(LS_ERROR) << "Error in replacing default auth module, error: "
                   << status;
     return err_status_fail;
   }
   return err_status_ok;
 }
 
 #endif  // defined(HAVE_SRTP) && defined(ENABLE_EXTERNAL_AUTH)