how_18_sees_it.rb

if $sitedir then
  if (not $sitedir.=~(Regexp.quote(version))) then
    $sitedir = File.join($sitedir, version)
  end
else
  $sitedir = File.join($libdir, "site_ruby")
end

source.rb

if !$sitedir
  $sitedir = File.join($libdir, "site_ruby")
elsif $sitedir !~ Regexp.quote(version)
  $sitedir = File.join($sitedir, version)
end