These variables should not be necessary, as BOOST_LIBRARYDIR is properly set.
It works as is on MacOS and Windows.
For some reason, on Linux, PCL still finds and uses the system Boost, ignoring these variables.
Boost_LIBRARY_DIR_DEBUG and Boost_LIBRARY_DIR_Release variables should be filled by CMake according to the BOOST_LIBRARYDIR hint.
By directly setting these variables, we ensure using the correct superbuild Boost libs.