[CMake] ExternalProject and redundant update/configure/compile actions.

[David Cole]

> Agreed, but right now ExternalProject is my favorite solution to
> this problem.  I prefer the ExternalProject approach over the
> ParaView + internal VTK / git-submodule approach.  So I'm
> just looking for ways to alter ExternalProject to be a little nicer
> to my workflow, which is incremental builds of multiple projects
> with project dependencies defined in a superbuild script.


Sure, and that sounds reasonable. Working with EP a little bit (with 
custom steps, or customized download/update/configure/build/install 
commands) to make your own workflow easier is reasonable.

I will continue to point out that expecting good incremental rebuild 
behavior from super builds is just silly, though.


D