Changes to support building on Mac OS X 10.6. When compiled on 10.6,

the resulting binary still works on 10.5.

lisp/Config.x86_darwin:
o Add -m32 flag to build and link a 32-bit binary.

lisp/Darwin-os.h:
o Include <sys/ucontext.h> instead of <ucontext.h> to get rid of the
  error about deprecated functions in ucontext.h.

lisp/gencgc.h:
o Mac OS X defines PAGE_SIZE in a header and that conflicts with our
  name.  Rename our PAGE_SIZE to GC_PAGE_SIZE.

lisp/gencgc.c:
o Rename PAGE_SIZE to GC_PAGE_SIZE.