USE_ and WANT_ for the Porter's Handbook

[Warren Block]

Here's a bare-bones pass at documenting the difference between USE_ and 
WANT_ variables.  WITH_ and WITHOUT_ have been removed because of the 
optionsng rework, and there is a Knobs section later in the PH that 
already describes them.

This section has been added to the end of the Dependencies section, 
currently 5.7.  It could be moved elsewhere.

http://www.wonkity.com/~wblock/porter/book.html
http://www.wonkity.com/~wblock/porter/usewant.diff