Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/expat.gyp

Index: build/linux/unbundle/expat.gyp
diff --git a/build/linux/unbundle/expat.gyp b/build/linux/unbundle/expat.gyp
new file mode 100644
index 0000000000000000000000000000000000000000..030fb856bb75da87f215d2ca6d501499ea855180
--- /dev/null
+++ b/build/linux/unbundle/expat.gyp
@@ -0,0 +1,17 @@
+# 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': 'expat',
+      'type': 'none',
+      'link_settings': {
+        'libraries': [
+          '-lexpat',
+        ],
+      },
+    },
+  ],
+}