Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/libxml.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': 'libxml',

[Ryan Sleevi, 2013/04/25 22:19:19]

Pretty sure you're going to want to name this separately, to avoid any naming
conflicts with the system library

[Paweł Hajdan Jr., 2013/04/25 22:41:50]

This is just extracting existing code that was in use. No such problem has been
observed.

If it ever becomes a problem, it'll be easy to fix in a separate CL.

[Paweł Hajdan Jr., 2013/04/26 00:55:18]

I tested it and it works. Here's a possible explanation: the library's name is
libxml2, while the target name is libxml.

+      'type': 'static_library',
+      'sources': [
+        'chromium/libxml_utils.h',
+        'chromium/libxml_utils.cc',
+      ],
+      'cflags': [
+        '<!@(pkg-config --cflags libxml-2.0)',
+      ],
+      'defines': [
+        'USE_SYSTEM_LIBXML',
+      ],
+      'direct_dependent_settings': {
+        'cflags': [
+          '<!@(pkg-config --cflags libxml-2.0)',
+        ],
+        'defines': [
+          'USE_SYSTEM_LIBXML',
+        ],
+      },
+      'link_settings': {
+        'ldflags': [
+          '<!@(pkg-config --libs-only-L --libs-only-other libxml-2.0)',
+        ],
+        'libraries': [
+          '<!@(pkg-config --libs-only-l libxml-2.0)',
+        ],
+      },
+    },
+  ],
+}