define( 'WP_SITEURL', 'http://hostname.com');
define( 'WP_HOME', 'http://hostname.com');

Or, to set it dynamically, to whatever domain name was used to access the site:

define( 'WP_SITEURL', 'http://' . $_SERVER['SERVER_NAME'] );
define( 'WP_HOME', 'http://' . $_SERVER['SERVER_NAME'] );