* src/reader.c (copy_comment2): New function, same as former

* src/reader.c (copy_comment2): New function, same as former
`copy_comment', but outputs into two FILE *.
(copy_comment): Use it.
(parse_union_decl): Use it.
(get_type, parse_start_decl): Use the same `invalid' message.
(parse_start_decl, parse_union_decl): Use the same `multiple'
message.
(parse_union_decl, copy_guard, copy_action): Use the same
`unmatched' message.
* m4/Makefile.am (EXTRA_DIST): Add `warning.m4'.