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
to False before calling "find_package(Slicer)"

In case of mismatch, the configure will be aborted displaying a message
similar to this one:

// --------------------
CMake Error at C:/D/N/Slicer-1-build/Slicer-build/SlicerConfig.cmake:475 (message):
  Mismatch between bitness of 'Foo' and 'Slicer' project !

        Slicer_CMAKE_SIZEOF_VOID_P:8

        CMAKE_SIZEOF_VOID_P:4

  Reconfigure 'Foo' project using a compatible Generator.

  Generator used to configure Slicer was: Visual Studio 9 2008 Win64

  See http://www.cmake.org/cmake/help/v2.8.11/cmake.html#opt:-Ggenerator-name
// --------------------

Co-authored-by: Julien Finet <julien.finet@kitware.com>