gentoo-overlay/app-emulation/spice/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
	<maintainer type="project">
		<email>virtualization@gentoo.org</email>
		<name>Gentoo Virtualization Project</name>
	</maintainer>
	<use>
		<flag name="smartcard">Enable smartcard remoting using <pkg>app-emulation/libcacard</pkg></flag>
	</use>
</pkgmetadata>