[CMake] dependencies in depend.make

[Daniel]

One thing I have noticed while trying to improve the speed of my builds is that 
the depends.make file for a project is fairly large (15MB) and that most of that 
size is dependencies against libraries I am using, mostly Boost and Qt.

It is fairly safe to assume that I won't be modifying these libraries, would 
removing these dependencies speed up my builds, and if so is there a way to tell 
cmake not to bother checking to see if certain directories have changed?
--
Daniel