Boost_LIBRARY_DIR_DEBUG and Boost_LIBRARY_DIR_Release variables should be automatically filled by CMake according to the BOOST_LIBRARYDIR hint. That's why these variables ettings have been removed in 7b70763c.
For some reason, on Linux, PCL still finds and uses the system Boost, ignoring the BOOST_LIBRARYDIR hint that is properly set. It already works as is on MacOS and Windows.
By directly setting these variables, we ensure using the correct superbuild Boost libs.