Build native library with BUCK for .NET (Mono, Xamarin)

Summary:
Examples:

- macOS on macOS (Mono, Xamarin.Mac, Unity)
  - `buck build //csharp:yoganet#shared,default` -> `libyoga.dylib`
- Android (Xamarin.Android, Unity)
  - `buck build //csharp:yoganet#shared,android-armv7` -> `libyoga.so`
- iOS (Xamarin.iOS, Unity)
  - `buck build //:yoga#static,iphoneos-arm64` -> `libyoga.a`
  - `buck build //csharp:yoganet#static,iphoneos-arm64` -> `libyoganet.a`
Closes https://github.com/facebook/yoga/pull/282

Reviewed By: emilsjolander

Differential Revision: D4333811

Pulled By: splhack

fbshipit-source-id: 54f4364d93b2419d7504392eb558296a5d06772f

csharp/BUCK

@@ -5,6 +5,10 @@
 # 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.
 
+include_defs('//YOGA_DEFS')
+
+COMPILER_FLAGS = BASE_COMPILER_FLAGS + ['-std=c++11']
+
 csharp_library(
   name = 'yogalibnet46',
   dll_name = 'Facebook.Yoga.dll',
@@ -18,3 +22,14 @@ csharp_library(
   framework_ver = 'net45',
   srcs = glob(['**/*.cs']),
 )
+
+cxx_library(
+  name = 'yoganet',
+  soname = 'libyoga.$(ext)',
+  srcs = glob(['Yoga/YGInterop.cpp']),
+  compiler_flags = COMPILER_FLAGS,
+  link_style = 'static',
+  link_whole = True,
+  deps = [yoga_dep(':yoga')],
+  visibility = ['PUBLIC'],
+)