Proposed fix for #773

Split up check_setup.php to do a check for the bare essentials (e.g. PHP version) first, then load the config files, etc. and do less important checks which may require access to config variables in the original check_setup.php. Patch fixes #773

Not sure if you like the way this is done, yet it seems to be a good way to get a hold of the configuration constants needed for the check.