Build : Implement BUILD_DIR like in Gaffer

[medubelko]

To build Cortex at home, users currently need to set many options; see the Configure section of the cmake instructions for a list.

It could be made significantly easier if users could set a single BUILD_DIR variable, which would point to GafferDependencies, just like the Gaffer build process. This would mean changing most of the paths in Sconstruct to point to that variable.