Python x64 and SCons Windows build support.

Dear Boost.NumPy team,

we are using Boost.NumPy and the ndarray project in a big data context at our lab. So I made the effort and made both projects x64 Python compatible, as well as the ndarrays 64bit indexed (similar to eigen3). I was mainly working on Windows and had to test the projects, so at the same time I extended the SCons build system to work with Windows.

After all changes, both projects build without any internal warnings (the numpy interface deprecation warning left aside) and are working flawlessly. I'll send pull requests for both, starting with Boost.NumPy.

Here, there wasn't much to do concerning x64 compatibility. The only related change is in boost/numpy/ndarray.hpp: the shape and stride of x64 numpy arrays are larger than int, and this can be adressed in general by using size_t.

The rest of the changes concerns building on Windows with SCons. I integrated the SConsChecks project (https://github.com/ChrislS/SConsChecks) and greatly simplified the SConscript. Apart from that, extension finding was simplified over tests and library files.

Best,
Christoph
I think the typedef we want to switch to here is Python's `Py_ssize_t`, not `std::size_t`.  I'm pretty sure the former is actually a signed type, but that's actually what we want, at least for strides (which can be negative).  And I think it makes sense to make the shape signed as well, to avoid having to constantly work around compiler warnings about signed/unsigned comparisons.