www-apache/mod_backtrace/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
<use>
  <flag name="unwind">
    Use <pkg>sys-libs/libunwind</pkg> to provide better resolution of
    function names.
  </flag>
</use>
</pkgmetadata>