Add NPY_CASTING
Add NPY_CASTING

Include `NPY_CASTING` along with the other things `extern`ed from
`numpy/arrayobject.h`.

da-woods/cython,scoder/cython,da-woods/cython,scoder/cython,da-woods/cython,cython/cython,cython/cython,cython/cython,scoder/cython,cython/cython,da-woods/cython,scoder/cython