PR for review https://github.com/mantisbt/mantisbt/pull/613

Maybe we could get rid of the whole config_eval stuff.
Not sure, but should be better in terms of performance and memory.

It seems to be used just a view times, e.g.
$g_cookie_prefix = 'MANTIS';
$g_string_cookie = '%cookie_prefix%_STRING_COOKIE';
$g_project_cookie = '%cookie_prefix%_PROJECT_COOKIE';
$g_manage_users_cookie = '%cookie_prefix%_MANAGE_USERS_COOKIE';
....

Do we really need this?
Isn't it enough to allow users to set just $g_cookie_prefix in config_defaults_inc.php and to compute the other values hardcoded on the fly?

I hardly believe that administrators do something like
$g_cookie_prefix = 'MyMANTIS';
$g_string_cookie = '%cookie_prefix%_MY_STRING_COOKIE';
$g_project_cookie = '%cookie_prefix%_MY_PROJECT_COOKIE';
...

@atrol, the idea was that the admin will just need to set one config option:
$g_cookie_prefix = 'MyMANTIS';

This will in-turn using eval result in the following values:
$g_string_cookie = 'MyMANTIS_STRING_COOKIE';
$g_project_cookie = 'MyMANTIS_PROJECT_COOKIE';

This also means that if we add a third one $g_xxx_cookie in a future release, then it will automatically pickup the same logical prefix.

Now whether this this is really useful or not is a different story :)