COMP: Compare Slicer bitness against extension bitness

On platform (i.e. Windows with Visual Studio) where it is possible to use
either a 32 or 64 bits generator, it is important to compare bitness
of Slicer project against project depending on Slicer (i.e. extension)
to ensure there are no mismatch.

The check could be disabled setting the variable Slicer_SKIP_CMAKE_SIZEOF_VOID_P_CHECK
before calling "find_package(Slicer)"