net-dialup/freeradius/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <use>
    <flag name="pcap">
      Build the RADIUS sniffer which requires <pkg>net-libs/libpcap</pkg>.
    </flag>
  </use>
</pkgmetadata>