Configure: New option --upgrade added

I have submitted the patch. Could you review it and let me know if it is right ? 
Method used : Basically while the upgrade option is enabled, the previous configuration files will not be deleted and thus the changes we make will still remain. Writing new contents to the configuration file will happen only when upgrade option is not enabled. With this, the creation of makefile still takes place. 

Note : There is seriously some problem with the indentation. I'm trying to figure out what it is. (It is fine in my text editor - vim). If the method is right, I will submit another one with the correct indentation. I'm going crazy correcting the indentation :-/