TFM based feature flags as compiler constants

Directory.Build.targets

<Project>

  <ItemGroup>
    <FeatureFlag Include="SUPPORTS_ECDSA" MinimumTargetFrameworks="netstandard1.6;net47" />
    <FeatureFlag Include="SUPPORTS_GENERIC_HOST" MinimumTargetFrameworks="netcoreapp2.2;netstandard2.1" />
    <FeatureFlag Include="SUPPORTS_SERVICE_PROVIDER_IN_HTTP_MESSAGE_HANDLER_BUILDER" MinimumTargetFrameworks="netcoreapp2.2;netstandard2.1" />
    <FeatureFlag Include="SUPPORTS_CERTIFICATE_HASHING_WITH_SPECIFIED_ALGORITHM" MinimumTargetFrameworks="netcoreapp2.1;netstandard2.1;net48" />
  </ItemGroup>

  <!--
    Use BeforeCompile here (instead of e.g. CoreCompile) so that the consants
    are picked up by _GenerateCompileDependencyCache and and incremental compilation
    picks up changes to the FeatureFlag items.
  -->
  <Target Name="CreateFeatureFlagCompilerConstants" BeforeTargets="BeforeCompile">
    <ItemGroup>
      <_FeatureFlagsByMinimumTargetFramework Include="%(FeatureFlag.MinimumTargetFrameworks)" 
                                             FeatureFlag="@(FeatureFlag)" />
      <_FeatureFlagsByMinimumTargetFramework Compatible="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)','%(Identity)'))" />
      <_CompatibleFeatureFlags Include="@(_FeatureFlagsByMinimumTargetFramework->WithMetadataValue('Compatible','True')->'%(FeatureFlag)'->Distinct())" />
    </ItemGroup>
    <PropertyGroup>
      <DefineConstants>$(DefineConstants);@(_CompatibleFeatureFlags)</DefineConstants>
    </PropertyGroup>
  </Target>
</Project>