Make update for updated packages

[Arthur Barlow <arthurbarlow%earthlink.net@localhost>]

What is the best way to "turn off" the building of upstream  
dependencies when you "make update" a particular package?  I've  
noticed that Firefox3 has many package dependencies and building it  
takes forever.  I like to not rebuild packages that depend on the  
package I'm building as most of them use dynamic libraries.

Art