Use IsOsPlatform

build/NetFX.props

@@ -1,6 +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="$([System.IO.File]::Exists('/usr/lib/libc.dylib')) ">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.6.1-api</FrameworkPathOverride>
+    <FrameworkPathOverride Condition="$([MSBuild]::IsOsPlatform('OSX'))">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.6.1-api</FrameworkPathOverride>
   </PropertyGroup>
 </Project>