jb::cpu_set is Linux specific

[coryan]

The jb::cpu_set class is Linux specific. It depends (and practically exposes) the implementation based on cpu_set_t and the CPU_* macros from <sched.h>. If we ever want to migrate this to other Unices or Windows we will need to define the class in terms on the native affinity cpumasks, and maybe even emulate it on platforms that lack them.