* src/scan-skel.l: Postprocess quadrigraphs.

* src/scan-skel.l: Postprocess quadrigraphs.
* src/reader.c (copy_character): New function, used to output
single characters while replacing `[' and `]' with quadrigraphs, to
avoid troubles with M4 quotes.
(copy_comment): Output characters with copy_character.
(read_additionnal_code): Likewise.
(copy_string2): Likewise.
(copy_definition): Likewise.
* tests/calc.at: Exercise M4 quoting.