Minor modifications to build on current OS X machine

I updated the path to the Carbon headers, which has changed with recent XCode versions. It is configurable by the CARBON_HEADERS environment variable. I also wrapped the BITS32/64 define in a check to see whether one is already defined, since BIT64 is set in Makefile.macosx. The check around __SIZEOF_POINTER__ was failing even when it was defined... not sure why. It happened GCC 4.2.1, GCC 4.7.2 and Clang 4.2 (LLVM 3.2).