Linux: fix header includes for system protobuf.

third_party/protobuf/protobuf.gyp

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'conditions': [
     ['use_system_protobuf==0', {
       'conditions': [
         ['OS!="win"', {
           'variables': {
@@ skipping 428 matching lines @@
         {
           'target_name': 'protobuf_lite',
           'type': 'none',
           'direct_dependent_settings': {
             'cflags': [
               # Use full protobuf, because vanilla protobuf doesn't have
               # our custom patch to retain unknown fields in lite mode.
               '<!@(pkg-config --cflags protobuf)',
             ],
             'defines': [
+              'USE_SYSTEM_PROTOBUF',
+
               # This macro must be defined to suppress the use
               # of dynamic_cast<>, which requires RTTI.
               'GOOGLE_PROTOBUF_NO_RTTI',
               'GOOGLE_PROTOBUF_NO_STATIC_INITIALIZER',
             ],
           },
           'link_settings': {
             # Use full protobuf, because vanilla protobuf doesn't have
             # our custom patch to retain unknown fields in lite mode.
             'ldflags': [
@@ skipping 10 matching lines @@
           'toolsets': ['host', 'target'],
         },
         {
           'target_name': 'py_proto',
           'type': 'none',
         },
       ],
     }],
   ],
 }