Check system boost environment variables and clean boost related CMake variables

If some Boost environment variables are defined, they could hide the superbuild Boost install. This can lead to superbuild failure if the system boost does not have all required components. It may also lead to runtime errors if all binaries were not built using the same boost libs (system VS superbuild ones).

This adds a check to verify the definition of these environment variables if user does not want to use his system boost install.

This also cleans a bit pcl.cmake by removing useless Boost related CMake variables :

• Boost_USE_STATIC which does not exist in CMake > 3.0.
• Boost_LIBRARY_DIR_DEBUG/RELEASE which are automatically set by FindBoost.cmake according to BOOST_LIBRARYDIR hint.

@nick.laurenson @jerome.dias @lea.vauchier @melanie.carriere PTAL