Support TBB 2021 version, which has backwards-incompatible changes
It seems TBB from the 2020 version to the 2021 version made a bunch of backwards-incompatible changes. The installed result has moved headers around, renamed them, and god knows what more. Notably the tbb_stddef.h file that VTK looks for is no longer there, and thus fails to build.
OpenCV had the same problem, recently fixed: https://github.com/opencv/opencv/issues/19358
The good news though is that TBB seems to have switched to using cmake, so that's nice at least!