Enable use of EnumeratorCancellation.

Ix.NET/Source/Directory.build.targets

@@ -27,6 +27,9 @@
   <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'netstandard2.1' or '$(TargetFramework)' == 'netstandard2.0'">
     <DefineConstants>$(DefineConstants);HAS_VALUETUPLE</DefineConstants>
   </PropertyGroup>
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'netstandard2.1'">
+    <DefineConstants>$(DefineConstants);HAS_ASYNC_ENUMERABLE_CANCELLATION</DefineConstants>
+  </PropertyGroup>
 
   <PropertyGroup>
     <Product>$(AssemblyName) ($(TargetFramework))</Product>