- only search for Boost if extra Boost components are required, - do not test `Boost_FOUND` (because Boost is `REQUIRED` in `find_package`), - use Boost imported targets instead of variable (compatibility with Boost-1.70 CMake config mode)
- only search for Boost if extra Boost components are required, - do not test `Boost_FOUND` (because Boost is `REQUIRED` in `find_package`), - use Boost imported targets instead of variable (compatibility with Boost-1.70 CMake config mode)