What is the preprocessor macro to test whether an application extension is being built?

Question

This question is based purely on publicly released documents regarding the introduction of application extensions in iOS.

With the introduction of app extensions in iOS

Answer 1

Update: Unfortunately, it doesn't actually work because it is working in __has_feature(attribute_availability_app_extension)-feature manner. Sad.

It is not actually that was asked but it is should be noted:

If you are using Swift, you have @available(iOSApplicationExtension) attribute! It is not actually preprocessor's feature but it is kind of compile time feature.

Example:

@available(iOSApplicationExtension, message="It is meaningless outside keyboard extension")
public var rootInputViewController: UIInputViewController {
    return storedInputViewController
}

Or with #-notation (but probably not):

public static var rootInputViewController: UIInputViewController! {
    guard #available(iOSApplicationExtension 8, *) else {
        return nil
    }

    return storedInputViewController!
}