Revert of Remove the rtc_relative_path GYP variable and similar defines

webrtc/pc/pc.gyp

 # 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.
 
 {
   'includes': ['../build/common.gypi'],
   'variables': {
     'rtc_pc_defines': [
+      'SRTP_RELATIVE_PATH',
       'HAVE_SCTP',
       'HAVE_SRTP',
     ],
   },
   'targets': [
     {
       'target_name': 'rtc_pc',
       'type': 'static_library',
       'dependencies': [
         '<(webrtc_root)/base/base.gyp:rtc_base',
@@ skipping 116 matching lines @@
               'sources': [
                 'rtc_pc_unittests.isolate',
               ],
             },
           ],
         }],
       ],  # conditions
     }],  # include_tests==1
   ],  # conditions
 }