gnome-extra/music-applet/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
	<herd>gnome</herd>
	<herd>sound</herd>
	<use>
		<flag name="mpd">Enable MPD support</flag>
	</use>
</pkgmetadata>