Make GConf dependency optional.

build/linux/system.gyp

 # Copyright (c) 2009 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': [
     ['sysroot!=""', {
       'variables': {
         'pkg-config': './pkg-config-wrapper "<(sysroot)"',
       },
@@ skipping 170 matching lines @@
             'libraries': [
               '<!@(<(pkg-config) --libs-only-l gdk-2.0)',
             ],
           },
       }]]
     },
     {
       'target_name': 'gconf',
       'type': 'settings',
       'conditions': [
-        ['_toolset=="target"', {
+        ['use_gconf==1 and _toolset=="target"', {
           'direct_dependent_settings': {
             'cflags': [
               '<!@(<(pkg-config) --cflags gconf-2.0)',
             ],
+            'defines': [
+              'USE_GCONF',
+            ],
           },
           'link_settings': {
             'ldflags': [
               '<!@(<(pkg-config) --libs-only-L --libs-only-other gconf-2.0)',
             ],
             'libraries': [
               '<!@(<(pkg-config) --libs-only-l gconf-2.0)',
             ],
           },
       }]]
@@ skipping 142 matching lines @@
       ],
     },
   ],
 }
 
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