x11: Include X11 dependencies only when building with X11.

build/linux/system.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.
 
 {
   'variables': {
     'conditions': [
       ['sysroot!=""', {
         'pkg-config': './pkg-config-wrapper "<(sysroot)" "<(target_arch)"',
       }, {
@@ skipping 730 matching lines @@
           },
           'link_settings': {
             'ldflags': [
               '<!@(pkg-config --libs-only-L --libs-only-other <(glib_packages))',
             ],
             'libraries': [
               '<!@(pkg-config --libs-only-l <(glib_packages))',
             ],
           },
         }],
-        ['chromeos==1', {
+        ['use_x11==1', {
           'link_settings': {
             'libraries': [ '-lXtst' ]
           }
         }],
       ],
     },
     {
       'target_name': 'pangocairo',
       'type': 'none',
       'toolsets': ['host', 'target'],
@@ skipping 55 matching lines @@
             ],
             'libraries': [
               '<!@(<(pkg-config) --libs-only-l libudev)',
             ],
           },
         }],
       ],
     },
   ],
 }