Linux: add option to use system nspr.

base/base_untrusted.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': {
     'chromium_code': 1,
   },
   'includes': [
     '../build/common_untrusted.gypi',
     'base.gypi',
   ],
   'conditions': [
     ['disable_nacl==0 and disable_nacl_untrusted==0', {
       'targets': [
         {
           'target_name': 'base_untrusted',
           'type': 'none',
           'variables': {
             'base_target': 1,
             'nacl_untrusted_build': 1,
             'nlib_target': 'libbase_untrusted.a',
             'build_glibc': 1,
             'build_newlib': 1,
             'sources': [
               'string16.cc',
               'sync_socket_nacl.cc',
-              'third_party/nspr/prtime.cc',
               'time_posix.cc',
             ],
           },
           'dependencies': [
             '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
           ],
         },
       ],
     }],
   ],
 }