Re-land "Extend the mDNS package with a native extension used on Mac OS"

src/pkg/mdns/mdns.gyp

+# Copyright (c) 2015, the Fletch project authors. Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE.md file.
+
+{
+  'targets': [
+    {
+      'target_name': 'mdns_extension_lib',
+      'type': 'shared_library',
+      'include_dirs': [
+        '../../../third_party/dart/runtime',
+      ],
+      'cflags!': [
+        '-Wnon-virtual-dtor',
+        '-Woverloaded-virtual',
+        '-fno-rtti',
+        '-fvisibility-inlines-hidden',
+        '-Wno-conversion-null',
+      ],
+      'sources': [
+        'mdns_extension.cc',
+        'mdns_extension_linux.cc',
+        'mdns_extension_macos.cc',
+      ],
+      'defines': [
+        # The only effect of DART_SHARED_LIB is to export the Dart API.
+        'DART_SHARED_LIB',
+      ],
+      'conditions': [
+        ['OS=="mac"', {
+          'xcode_settings': {
+            'OTHER_LDFLAGS': [ '-undefined', 'dynamic_lookup' ],
+          },
+        }],
+        ['OS=="linux"', {
+          'cflags': [
+            '-fPIC',
+          ],
+        }],
+      ],
+    },
+  ],
+}