32-bit build is selected from the CMake command line

A CMake command line option rather than a build option is required to select 32-bit builds.  The readme file has been updated correspondingly.  Doing this allows CMake to correctly set variables such as CMAKE_SIZEOF_VOID_P and ensures that the behavior of find_package and find_library is correct.
Merge pull request #203 from leapmotion/fix-32bitbuild

32-bit build is selected from the CMake command line