Back out thread.h changes from a while back -- they had unintentional
performance degredation.  It was incorrect to force only one of
USE_GCC_ATOMICS and USE_TBB to be true; there were some places (such
as atomic_compare_and_exchange where the gcc intrinsics were used if
available, even if TBB was being used, and apparently the gcc intrinsics
were much faster.