dev-lua/messagepack/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <maintainer type="person">
    <email>yngwin@gentoo.org</email>
    <name>Ben de Groot</name>
  </maintainer>
  <use>
    <flag name="luajit">
      Use the lua just-in-time compiler <pkg>dev-lang/luajit</pkg> instead of
      <pkg>dev-lang/lua</pkg>
    </flag>
  </use>
</pkgmetadata>