Removed rdtsc fallback from bt_monotonic_now().
Fixed incorrect prefix in Makefile.
Added bt/foundation/compat/attributes.h.
Added bt_rdtsc().
bt_profile_begin() and bt_profile_end() now record the inclusive number of cycles taken.
bt_profile_begin() and bt_profile_end() now record the exlcusive number of cycles.
Fixed by_cycles_to_{sec,msec,usec} conversions.