Add basic Xamarin.Mac support

Summary:
- Does not contain "magic" found in YogaKit yet, but enough to get started
- Simple test project showing use
- YGInteropSetLogger and YGNodeInit(IntPtr) were missing from native lib built by buck
Closes https://github.com/facebook/yoga/pull/278

Reviewed By: emilsjolander

Differential Revision: D4388480

Pulled By: splhack

fbshipit-source-id: a7387bb5b5554b6fce80d08c23e4fa18a4611cce

csharp/Mac/ApiDefinition.cs

@@ -0,0 +1,23 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+using System;
+
+using AppKit;
+using Foundation;
+using ObjCRuntime;
+using CoreGraphics;
+
+namespace Facebook.Yoga.Mac
+{
+    // Xamarin.Mac binding projects allow you to include native libraries inside C# DLLs for easy consumption
+    // later. However, the binding project build files currently assume you are binding some objective-c API
+    // and that you need an ApiDefinition.cs for that. yoga is all C APIs, so just include this "blank" file so
+    // the dylib gets packaged
+}