Check for Boost environment variables hiding superbuild Boost install
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 commit add a check to verify the definition of these environment variables if user does not want to use his system boost install.