I'm not sure where the site_url is defined, but you can override this setting by adding the below line to the file "public_html/siteconfig.php" (somewhere in the middle, not at the very end)
Text Formatted Code
$_CONF['site_url'] = 'http://mysite.com/';
But the best way to define the site url is during the step 2 of the install, in the last and second last text inputs.
Rouslan