Allow configuration values to be rearranged in config_defaults

Within config_defaults_inc.php, the following syntax doesn't allow for
configuration values to be rearranged within the file:
$g_some_value = $g_something_else

Instead we should use the following syntax where references to other
values (anywhere in the file, even if not declared yet) are resolved
after the entire config_defaults_inc.php file is loaded:
$g_some_value = '%something_else%'