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

[David Cole]

> Some background about the Slicer build system that inspired
> the NamicExternalProjects system discussed by Kent  ... 
>
> Before CMake 2.8.8, we disabled the UPDATE of external project
> because it was not working well with Visual Studio. While this was
> working as expected, the overall build time was also increased
> because it wasn't doing an incremental rebuild.

The build time increased compared to what?

A second build, in a no update situation, should yield "no work to do" 
because nothing got updated.


> As of today, we re-enabled the UPDATE part of external project
> and also require CMake >= 2.8.9 and it works quite well. The
> overall build time is largely decreased.

Again, I don't understand how updating as part of the superbuild could 
ever result in a decreased build time...


What am I missing here?
D