<Project DefaultTargets="Build">

  <Target Name="A" Condition="$(X) == 'true'">
    <Message Text="Target A runs" Importance="high" />
  </Target>

  <Target Name="B" DependsOnTargets="A">
  </Target>

  <Target Name="C" >
    <PropertyGroup>
      <X>true</X>
    </PropertyGroup>
  </Target>

  <Target Name="D" DependsOnTargets="A">
  </Target>

  <Target Name="Build" DependsOnTargets="B;C;D">
  </Target>

</Project>