Rpm packaging

These changes allow me to build both 32-bit and 64-bit RPMs for my Centos box.  They comprise:
-  fixing up 32-bit build warnings
-  make sure that 64-bit libraries go in lib64 (per #519)
-  introduction of CMake variable 32BIT, to allow user to force 32-bit compile
-  introduction of CMake variable BUILD_STATIC_LIB, to control whether or not to bother building static libraries (I don't want them, but I've left default as ON since that's what the current behaviour is)
-  use CPack to build the rpm