libcpp:
* include/cpplib.h (struct cpp_callbacks): Add used_define,
used_undef and before_define.
(NODE_USED): Define.
* directives.c (do_define, do_undef, undefine_macros, do_ifdef,
do_ifndef, cpp_pop_definition): Handle new flag and use new
callbacks.
* expr.c (parse_defined): Handle new flag and use new callbacks.
* macro.c (enter_macro_context, _cpp_free_definition): Handle new
flag and use new callbacks.