Add _SC_NPROCESSORS_ONLN constants for linux and macos

[oh-its-jimjam]

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.

seanmonstar/num_cpus#2