Create NetFX.props

[skip ci]

build/NetFX.props

@@ -0,0 +1,6 @@
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'net461' and '$(OS)' == 'Unix' ">
+    <FrameworkPathOverride>/usr/lib/mono/4.6.1-api</FrameworkPathOverride>
+    <FrameworkPathOverride Condition="$([MSBuild]::IsOsPlatform('OSX'))">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.6.1-api</FrameworkPathOverride>
+  </PropertyGroup>
+</Project>