Make FindVDT.cmake compatible with ROOT's FindVdt.cmake

The builds in lhcb-lcg-dev4lhcb are failing because we (implicitly) end up using a mix of FindVdt.cmake from ROOT and FindVDT.cmake from LHCb.

I modified FindVDT.cmake to avoid destructive interference with ROOT version.

An alternative fix would be to use directly FindVdt.cmake from ROOT, but that requires a more recent version of ROOT or to copy the file in LHCb and to replace all uses of VDT::vdt with VDT::vdt.