Use sysconf on macOS to determine number of CPUs

[moshegottlieb]

Added #if defined(__APPLE__) wrapper for count_processors, using sysconf (in libc, include unistd.h which is already indirectly included). sysconf(_SC_NPROCESSORS_ONLN) returns the number of processors currently online or -1 if there was an error, this function uses -1 to denote an error so this works fine.