Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/libpng.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': 'libpng',
+      'type': 'none',
+      'dependencies': [
+        '../zlib/zlib.gyp:zlib',

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

Are you sure this is correct? This is stating that if you use libpng, you MUST
use system zlib.

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

This is not the case. What makes you think so?

[Ryan Sleevi, 2013/04/25 23:12:40]

Apologies, I misread the path of this file to think it was
build/linux/unbundle/libpng/libpng.gyp (eg: that ../ still kept you in
build/linux/unbundle)

+      ],
+      'direct_dependent_settings': {
+        'cflags': [
+          '<!@(pkg-config --cflags libpng)',
+        ],
+      },
+      'link_settings': {
+        'ldflags': [
+          '<!@(pkg-config --libs-only-L --libs-only-other libpng)',
+        ],
+        'libraries': [
+          '<!@(pkg-config --libs-only-l libpng)',
+        ],
+      },
+      'variables': {
+        'headers_root_path': '.',
+        'header_filenames': [
+          'png.h',
+          'pngconf.h',
+        ],
+      },
+      'includes': [
+        '../../build/shim_headers.gypi',
+      ],
+    },
+  ],
+}