Update Directory.Build.props

Author: mythz

src/Directory.Build.props

@@ -24,24 +24,27 @@
     <Optimize>true</Optimize>
   </PropertyGroup>
 
-  <PropertyGroup Condition=" '$(TargetFramework)' == 'net45' ">
-    <DefineConstants>$(DefineConstants);NET45</DefineConstants>
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'net45' OR '$(TargetFramework)' == 'net451' OR '$(TargetFramework)' == 'net452' OR '$(TargetFramework)' == 'net46'  OR '$(TargetFramework)' == 'net472' ">
+    <DefineConstants>$(DefineConstants);NETFX;NET45</DefineConstants>
     <SignAssembly>True</SignAssembly>
     <DelaySign>False</DelaySign>
     <AssemblyOriginatorKeyFile>../servicestack.snk</AssemblyOriginatorKeyFile>
-    <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
+  </PropertyGroup>
+
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'net472' ">
+    <DefineConstants>$(DefineConstants);NETFX;NET472</DefineConstants>
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
-    <DefineConstants>$(DefineConstants);NETSTANDARD2_0</DefineConstants>
+    <DefineConstants>$(DefineConstants);NETSTANDARD;NETSTANDARD2_0</DefineConstants>
+  </PropertyGroup>
+
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.1' ">
+    <DefineConstants>$(DefineConstants);NETSTANDARD;NETSTANDARD2_1</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
-    <PackageReference Include="Microsoft.NetFramework.ReferenceAssemblies" Version="1.0.0-preview.2">
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-      <PrivateAssets>all</PrivateAssets>
-    </PackageReference>
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">