Make sure the 'use_system_*' variables are actually visible in the relevant gyp files.

third_party/bzip2/bzip2.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.
 
 {
   'variables': {
-    'use_system_bzip2%': 0,
+    'conditions': [
+      [ 'OS=="linux"', {
+        # Link to system .so since we already use it due to GTK.
+        'use_system_bzip2%': 1,
+      }, {  # OS!="linux"
+        'use_system_bzip2%': 0,
+      }],
+    ],
   },
   'conditions': [
     ['use_system_bzip2==0', {
       'targets': [
         {
           'target_name': 'bzip2',
           'type': '<(library)',
           'defines': ['BZ_NO_STDIO'],
           'msvs_guid': '2A70CBF0-847E-4E3A-B926-542A656DC7FE',
           'sources': [
@@ skipping 43 matching lines @@
       ]
     }],
   ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: