Allow C++ compilation on Linux, as a safety measure in type

Allow C++ compilation on Linux, as a safety measure in type
checking.
* m4/gnulib-cache.m4: Augment with 'gnulib-tool --import
stdbool'.
* src/m4.h (hack_symbol, hack_all_symbols): Use full prototype.
(boolean): Kill this, and use stdbool.h instead.
* src/debug.c, src/eval.c, src/input.c, src/macro.c, src/m4.c:
* src/output.c, src/symtab.c: All users changed.
* src/symtab.c (hack_all_symbols): Update prototype.
* src/builtin.c (dump_symbol, set_trace): Update signature.
(m4_dumpdef, m4_traceon, m4_traceoff): Update callers.
(mkstemp_helper, m4_m4wrap, expand_ranges, m4_translit): Allow
C++ compilation.
* src/debug.c (trace_flush): Likewise.
* src/freeze.c (reload_frozen_state): Likewise.
* src/input.c (push_file, push_string_finish, push_wrapup):
(token_bottom, next_token): Likewise.
* src/m4.c (main): Likewise.