Complete Boost.Build infrastructure for NumPy integration.

[stefanseefeld]

I just merged a NumPy extension from https://github.com/ndarray/Boost.NumPy, such that it is built and tested using the SCons-based build system.
This still lacks integration with the Boost.Build infrastructure, such that it can be built and tested as part of the rest of Boost, too.

[stefanseefeld]

A partial implementation is here:
stefanseefeld@5869328
This works fine on my Linux laptop, but needs some cleanup...

[stefanseefeld]

A partial implementation is here:
stefanseefeld@5869328
This works fine on my Linux laptop, but needs some cleanup...