Gem::Commands::SetupCommand#generate_default_man_dir

# File tmp/rubies/ruby-3.4.1/lib/rubygems/commands/setup_command.rb, line 429
def generate_default_man_dir
  prefix = options[:prefix]

  if prefix.empty?
    man_dir = RbConfig::CONFIG["mandir"]
    return unless man_dir
  else
    man_dir = File.join prefix, "man"
  end

  prepend_destdir_if_present(man_dir)
end