The value is not asked for in configuration.php and isn't set within the database as part of running install.php.
I've added patches below which modify configuration.php and install.php.
Zip file containing two patches. One that provides user access to the email_system variable for setting the value and another that modifies install.php to set the value during installation.
I originally uploaded a bad patch, but I've since confirmed both cleanly apply and have placed them both in a zip file that contains the ticket number in the name.