(rescan): Add support for %: and %:%: digraphs.

(collect_expansion, skip_if_group, dump_single_macro): Likewise.
(union hashval): Remove ival member; it doesn't suffice for `199409L'.
(special_symbol): Use cpval instead of ival for T_CONST macros.
(install): Remove ivalue argument, since string value now suffices.
All callers changed.
(initialize_builtins): Switch from int to string values for T_CONST.
If not traditional, initialize __STDC_VERSION__ to 199409L.
(STDC_VALUE): Remove no-longer-used configuration macro.

From-SVN: r8983