Simpler variant of commit r14635:
- configure: no change necessary
- byterun/config.h: make sure ARCH_INT64_TYPE and related macros
  are always defined, for Coq and others to use.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14636 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02