Linux: add option to use system nspr.

build/shim_headers.gypi

 # 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.
 
 # This file is meant to be included into a target to handle shim headers
 # in a consistent manner. To use this the following variables need to be
 # defined:
 #   headers_root_path: string: path to directory containing headers
 #   header_filenames: list: list of header file names
 
 {
   'variables': {
-    'shim_headers_path': '<(SHARED_INTERMEDIATE_DIR)/shim_headers/<(_target_name)',
+    'shim_headers_path': '<(SHARED_INTERMEDIATE_DIR)/shim_headers/<(_target_name)/<(_toolset)',
   },
   'include_dirs+++': [
     '<(shim_headers_path)',
   ],
   'direct_dependent_settings': {
     'include_dirs++++': [
       '<(shim_headers_path)',
     ],
   },
   'actions': [
@@ skipping 15 matching lines @@
       ],
       'action': ['python',
                  '<(generator_path)',
                  '<@(generator_args)',
                  '--generate',
       ],
       'message': 'Generating <(_target_name) shim headers.',
     },
   ],
 }