Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/opus.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': 'opus',
+      'type': 'none',
+      'direct_dependent_settings': {
+        'cflags': [
+          '<!@(pkg-config --cflags opus)',
+        ],
+      },
+      'variables': {
+        'headers_root_path': 'src/include',
+        'header_filenames': [
+          'opus_custom.h',
+          'opus_defines.h',
+          'opus_multistream.h',
+          'opus_types.h',
+          'opus.h',
+        ],
+      },
+      'includes': [
+        '../../build/shim_headers.gypi',
+      ],
+      'link_settings': {
+        'ldflags': [
+          '<!@(pkg-config --libs-only-L --libs-only-other opus)',
+        ],
+        'libraries': [
+          '<!@(pkg-config --libs-only-l opus)',
+        ],
+      },
+    },
+  ],
+}