Update Build.props

Author: mythz

src/Directory.Build.props

@@ -43,6 +43,14 @@
     <DefineConstants>$(DefineConstants);NETSTANDARD;NETSTANDARD2_1</DefineConstants>
   </PropertyGroup>
 
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'net5.0' ">
+    <DefineConstants>$(DefineConstants);NET50</DefineConstants>
+  </PropertyGroup>
+
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == 'netstandard2.1' OR '$(TargetFramework)' == 'netcoreapp3.1' OR '$(TargetFramework)' == 'net5.0' ">
+    <DefineConstants>$(DefineConstants);NETCORE_SUPPORT;NETCORE</DefineConstants>
+  </PropertyGroup>
+
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
   </ItemGroup>