Fix condition on which to get the TBB version

The FindTBB.cmake module gets the TBB version by reading and grepping one of the header files. Before doing so, it checks to see if the version variable already exists. The problem was it was checking TBB_VERSION, which was never created. Worse, other build systems might set this for us (for example, the ParaView superbuild). Correct this by making sure both TBB_VERSION_MAJOR and TBB_VERSION_MINOR are set.