generate_default_dirs

# File lib/rubygems/commands/setup_command.rb, line 341
  def generate_default_dirs(install_destdir)
    prefix = options[:prefix]
    site_or_vendor = options[:site_or_vendor]

    if prefix.empty? then
      lib_dir = RbConfig::CONFIG[site_or_vendor]
      bin_dir = RbConfig::CONFIG['bindir']
    else
      # Apple installed RubyGems into libdir, and RubyGems <= 1.1.0 gets
      # confused about installation location, so switch back to
      # sitelibdir/vendorlibdir.
      if defined?(APPLE_GEM_HOME) and
        # just in case Apple and RubyGems don't get this patched up proper.
        (prefix == RbConfig::CONFIG['libdir'] or
         # this one is important
         prefix == File.join(RbConfig::CONFIG['libdir'], 'ruby')) then
         lib_dir = RbConfig::CONFIG[site_or_vendor]
         bin_dir = RbConfig::CONFIG['bindir']
      else
        lib_dir = File.join prefix, 'lib'
        bin_dir = File.join prefix, 'bin'
      end
    end

    unless install_destdir.empty? then
      lib_dir = File.join install_destdir, lib_dir.gsub(/^[a-zA-Z]:/, '')
      bin_dir = File.join install_destdir, bin_dir.gsub(/^[a-zA-Z]:/, '')
    end

    [lib_dir, bin_dir]
  end