app-crypt/gnupg/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>crypto</herd>
  <longdescription>
    GnuPG is a complete and free replacement for PGP. Because it does
    not use the patented IDEA algorithm, it can be used without any
    restrictions. GnuPG is a RFC2440 (OpenPGP) compliant application.
  </longdescription>
  <use>
    <flag name='smartcard' restrict='&lt;app-crypt/gnupg-2.0.17-r1'>
      Bring in <pkg>dev-libs/libusb</pkg> as a dependency; enable
      scdaemon.
    </flag>
    <flag name='smartcard' restrict='&gt;=app-crypt/gnupg-2.0.17-r1'>
      Build scdaemon software. Enables usage of OpenPGP cards. For
      other type of smartcards, try
      <pkg>app-crypt/gnupg-pkcs11-scd</pkg>.
    </flag>
    <flag name='usb' restrict='&gt;=app-crypt/gnupg-2.0.17-r1'>
      Build direct CCID access for scdaemon; requires
      <pkg>dev-libs/libusb</pkg>.
    </flag>
    <flag name='mta'>
      Build mta support using
      <pkg>virtual/mta</pkg>.
    </flag>
  </use>
</pkgmetadata>