Linux: include system Mesa headers when using system Mesa.

third_party/mesa/mesa.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': {
     # Disable warnings as errors for mesa until they're fixed or disabled.
     # http://crbug.com/143877
     'win_third_party_warn_as_error': 'false',
 
@@ skipping 574 matching lines @@
             'MesaLib/src/mesa/drivers/osmesa/osmesa.def',
           ],
         },
       ],
     }, { # use_system_mesa==1
       'targets': [
         # TODO(phajdan.jr): Make this work, http://crbug.com/161389 .
         {
           'target_name': 'mesa_headers',
           'type': 'none',
+          'direct_dependent_settings': {
+            'defines': [
+              'USE_SYSTEM_MESA',
+            ],
+          }
         },
         {
           'target_name': 'mesa',
           'type': 'none',
         },
         {
           'target_name': 'osmesa',
           'type': 'none',
         },
       ],
     }],
   ],
 }