properly check for write permissions to the config file

lib/util.php

@@ -226,6 +226,10 @@ class OC_Util {
 			$errors[]=array('error'=>'PHP module ctype is not installed.<br/>','hint'=>'Please ask your server administrator to install the module.');
 		}
 
+		if(!is_writeable(OC::$SERVERROOT."/config/config.php")){
+			$errors[]=array('error'=>"Can't write into config directory 'config'",'hint'=>"You can usually fix this by giving the webserver use write access to the config directory in owncloud");
+		}
+
 		return $errors;
 	}