Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/re2.gyp

+# Copyright 2013 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.
+
+{
+  'targets': [
+    {
+      'target_name': 're2',
+      'type': 'none',
+      'variables': {
+        'headers_root_path': '.',
+        'header_filenames': [
+          're2/filtered_re2.h',
+          're2/re2.h',
+          're2/set.h',
+          're2/stringpiece.h',
+          're2/variadic_function.h',
+        ],
+        'shim_generator_additional_args': [
+          # Chromium copy of re2 is patched to rename POSIX to POSIX_SYNTAX
+          # because of collision issues that break the build.
+          # Upstream refuses to make changes:
+          # http://code.google.com/p/re2/issues/detail?id=73 .
+          '--define', 'POSIX=POSIX_SYNTAX',
+        ],
+      },
+      'includes': [
+        '../../build/shim_headers.gypi',
+      ],
+      'link_settings': {
+        'libraries': [
+          '-lre2',
+        ],
+      },
+    }
+  ],
+}