This continues the cmake modernization effort and introduces support for
allowing our optional dependencies to integrate properly. TBB is added
here as it's proven troublesome to maintain correctly.
Currently the only Blender project which uses the TBB headers directly
is `blenlib`. However, all downstream projects which require blenlib as
their dependency, and wish to properly make use of its threading
facilities, needed to define various TBB items in their CMake files. Not
only is this unnecessary and arcane, but several projects didn't do this
and ended up not using threading as well as producing ODR violations
along the way[1].
This PR makes TBB a modern dependency and exposes it PUBLIC'ly from
`blenlib`. All downstream projects which depend on blenlib will now
receive everything they require from TBB automatically. This includes
the `WITH_TBB` define, the headers, and the library itself.
[1] blender/blender@05241f47f5
Pull Request: https://projects.blender.org/blender/blender/pulls/124916
ec4fc2d34a