Detect target framework version at compile time

I have some code which makes use of Extension Methods, but compiles under .NET 2.0 using the compiler in VS2008. To facilitate this, I had to declare ExtensionAttribute:

/// <summary>
/// ExtensionAttribute is required to define extension methods under .NET 2.0
/// </summary>
public sealed class ExtensionAttribute : Attribute
{
}

However, I'd now like the library in which that class is contained to also be compilable under .NET 3.0, 3.5 and 4.0 - without the 'ExtensionAttribute is defined in multiple places' warning.

Is there any compile time directive I can use to only include the ExtensionAttribute when the framework version being targetted is .NET 2?