Something

    local backend
	if use jack ; then
        backend="jack"
	elif use alsa ; then
		backend="alsa"
	elif use oss ; then
        backend="oss"
	else
		einfo "One of the following USE flags is needed: jack, alsa or oss"
		die "One of the following USE flags is needed: jack, alsa or oss"
	fi
    emake "linux-${backend}" || die "emake failed"