gentoo-overlay/dev-ruby/nokogiri/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>ruby</herd>
  <use>
    <flag name="ffi">
      Use the Foreign Function Interface library (as provided by
      <pkg>dev-java/jruby</pkg> or <pkg>dev-ruby/ffi</pkg> depending
      on the implementation) rather than building a C extension to
      wrap around the libxml/libxslt interfaces. Note: JRuby always
      uses the FFI interface as native extensions are not supported.
    </flag>
  </use>
</pkgmetadata>