Incremental build?

[Frank Solensky <frank solensky org>]

As my build of 2.13.4 enters its sixth hour on an old portable, the
half-baked thought strikes me: would it be possible to set up a build so
that only modified files get built?

I'm thinking something along the lines of the following:

$ cp -al garnome-2.13.3 garnome-2.13.4
$ tar -xjvf garnome-2.13.4.tar.bz2

##  make sure "gar.*" files are set up correctly 

$ cd desktop
$ make paranoid-install

TIA..