[PATCH] clctypes.h: Don't rely on stddef.h for size_t and ptrdiff_t

Fixes the build and run-time for me with llvm 3.5.0 git/svn

Tested-by: Aaron Watry <awatry@gmail.com>

I believe we loose access to NULL in this way, or not?

Although NULL is not officially in the standard, it might be useful to have it around?