Extract use_system_foo logic out of gyp files, patch 2

build/linux/unbundle/zlib.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': 'zlib',
+      'type': 'none',
+      'variables': {
+        'headers_root_path': '.',
+        'header_filenames': [
+          'zlib.h',
+        ],
+      },
+      'includes': [
+        '../../build/shim_headers.gypi',
+      ],
+      'direct_dependent_settings': {
+        'defines': [
+          'USE_SYSTEM_ZLIB',
+        ],
+      },
+      'link_settings': {
+        'libraries': [
+          '-lz',
+        ],
+      },
+    },
+    {
+      'target_name': 'minizip',
+      'type': 'static_library',
+      'all_dependent_settings': {
+        'defines': [
+          'USE_SYSTEM_MINIZIP',
+        ],
+      },
+      'defines': [
+        'USE_SYSTEM_MINIZIP',
+      ],
+      'link_settings': {
+        'libraries': [
+          '-lminizip',
+        ],
+      },
+    }
+  ],
+}