[CMake] Best way to build Insight as a subdirectory of a larger project

[Kent Williams]

I'm trying to make our application brains2 a CMake project.  In order to 
build brains2 there are dependencies on several open source projects, 
including Insight (www.itk.org).

I'd like to build Insight as a subproject of brains2, which raises 2 
questions:

1. Insight's CMakeLists.txt file is written to be a 'top-level' CMake 
project. I'd like to use the regular CMakeLists.txt
file but pass down from the brains2 level things like target directories 
and compiler flags. Currently we automate that
by running cmake in the brains2 makefile.

2. The current CMakeLists.txt for the core of brains2 tries to find an 
existing build of Insight.  If I'm configuring brains2, and Insight 
isn't yet built, what's the best way to give the required paths to 
library and include files, which won't exist until Insight is configured 
and built?