Use TBB_IMPORTED_TARGETS for TBB CMake configuration

Ever since TBB has started providing built-in CMake support in 2017 U7,
TBB_IMPORTED_TARGETS has been the only supported way to link to TBB
using CMake. In particular, the official TBB CMake files do not export
TBB_INCLUDE_DIR and TBB_LIBRARY variables.

See https://github.com/intel/tbb/blob/tbb_2020/cmake/README.rst for
more details about TBB's official CMake support.