Linux: add option to use system nspr.

base/third_party/nspr/nspr.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.
+
+{
+  'conditions': [
+    ['use_system_nspr==1', {
+      'targets': [
+        {
+          'target_name': 'nspr',
+          'type': 'none',
+          'toolsets': ['host', 'target'],
+          'variables': {
+            'headers_root_path': '.',
+            'header_filenames': [
+              'prcpucfg.h',
+              'prtime.h',
+              'prtypes.h',
+            ],
+          },
+          'includes': [
+            '../../../build/shim_headers.gypi',
+          ],
+          'direct_dependent_settings': {
+            'cflags': [
+              '<!@(pkg-config --cflags nspr)',
+            ],
+          },
+          'link_settings': {
+            'ldflags': [
+              '<!@(pkg-config --libs-only-L --libs-only-other nspr)',
+            ],
+            'libraries': [
+              '<!@(pkg-config --libs-only-l nspr)',
+            ],
+          },
+        }
+      ],
+    }],
+  ],
+}