gentoo-overlay/sys-apps/pcsc-lite/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>crypto</herd>
  <longdescription>
    PC/SC Architecture smartcard middleware library
  </longdescription>

  <use>
    <flag name='libusb' restrict="&gt;=sys-apps/pcsc-lite-1.8.0">
      Use <pkg>dev-libs/libusb</pkg> detection to hotplug new
      smartcard readers. This flag should only be enabled if you're
      running a non-Linux kernel or you don't want to use
      <pkg>sys-fs/udev</pkg>.
    </flag>

    <flag name='udev'>
      Use <pkg>sys-fs/udev</pkg> rules to handle devices' permissions
      and hotplug support. Unless you know what you're doing do not
      disable this flag on Linux kernels. This is provided as an
      option for completeness.
    </flag>
  </use>
</pkgmetadata>