[CMake] add_subdirectory and build directory

[Bill Hoffman]

> 
> That's NOT what add_subdirectory is made for. It is intended for adding 
> a sub-directory in the source tree. So, if your directory structure 
> looks like this (i.e. B is a sub-project of A)
> 
> A/CMakeLists.txt
> A/B/CMakeLists.txt
> 
> things are simple:
> 
> A/CMakeLists.txt:

Actually add_subdirectory does support the notion of out-of-source 
source.   So, a source directory in does not have to be a direct 
sub-directory on disk.

-Bill