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

third_party/libxslt/libxslt.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': {
     'conditions': [
       ['OS=="linux" or OS=="freebsd"', {'os_include': 'linux'}],
       ['OS=="mac"', {'os_include': 'mac'}],
       ['OS=="win"', {'os_include': 'win32'}],
+      [ 'OS=="linux"', {
+        # Link to system .so since we already use it due to GTK.
+        'use_system_libxslt%': 1,
+      }, {  # OS!="linux"
+        'use_system_libxslt%': 0,
+      }],
     ],
-    'use_system_libxslt%': 0,
   },
   'targets': [
     {
       'target_name': 'libxslt',
       'conditions': [
         ['OS=="linux" and use_system_libxslt', {
           'type': 'settings',
           'direct_dependent_settings': {
             'cflags': [
               '<!@(pkg-config --cflags libxslt)',
@@ skipping 83 matching lines @@
       ],
     },
   ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: