gentoo-overlay/media-plugins/gst-plugins-ffmpeg/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
	<herd>gstreamer</herd>
	<use>
		<flag name="orc">Use <pkg>dev-lang/orc</pkg> for runtime
			optimisations</flag>
    </use>
</pkgmetadata>