Always set TBB_LIBRARIES when TBB_FOUND
@clemenci
This fixes the problem appearing in lhcb-gaussino
slot built with LCG 99 and dd4hep.
Here, https://gitlab.cern.ch/gaudi/Gaudi/-/blob/master/cmake/modules/FindTBB.cmake#L123, TBB_LIBRARIES won't be set provided that TBB was found at some point before. This results in https://gitlab.cern.ch/LHCbOpt/SOAContainer/-/blob/master/examples/nbody/CMakeLists.txt#L15 not linking with tbb.