// (need == needVet, not testing just the one bit).
	// If we are going to do a full build anyway,
	// we're going to regenerate the files below anyway.
	if need == needVet {
		if err := b.loadCachedVet(a); err == nil {
			need &^= needVet
		}
	}
	if need == 0 {
		return nil
	}

	if err := AllowInstall(a); err != nil {
		return err
	}

	// make target directory