[CMake] Build only what you need in third party libs

[troy d. straszheim]

Michael Jackson wrote:
> 
> So you are wanting to include the Boost sources in your project and you 
> just want to build a specific subset of Boost to use with your project?
> 

Here's what I came up with:

http://sodium.resophonic.com/boost-cmake/current-docs/exported_targets.html#with-boost-source-in-a-subdirectory-of-your-source

the EXCLUDE_FROM_ALL option to add_subdirectory makes things remarkably 
easy... nice feature.

-t