Although it is not anymore the base build system, adding tbb support was
not too complicated and allows still easy testing of this new feature.
Use ./configure --enable-tbb. TBB needs to be preinstalled, there is no
support for the on the fly TBB installation featured supported by cmake.

Removed option 'tbb'. Enable TBB when using the option 'imt'. Inverted the logic to unable 'builtin_tbb' if TBB is not found in the system.