Add _SC_NPROCESSORS_ONLN constants for linux and macos

There is a bug in `num_cpu` where the call to `sysconf` is hard-coded to `84` so doesn't work when on Mac OS X; which consequently brakes a lot of libraries when used on Mac OS X.

We need to add constants to `numcpu` so this type of bug doesn't occur again.

https://github.com/seanmonstar/num_cpus/issues/2
