[InterNetNews/inn] 97e9cd: Improve M4 style

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: 97e9cdcdf8d8696b665634507d2503eaecb10806
      https://github.com/InterNetNews/inn/commit/97e9cdcdf8d8696b665634507d2503eaecb10806
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2022-12-24 (Sat, 24 Dec 2022)

  Changed paths:
    M m4/aux-libs.m4
    M m4/cc-c-o.m4
    M m4/compress.m4
    M m4/iov-max.m4
    M m4/large-fpos.m4
    M m4/mmap.m4
    M m4/modes.m4
    M m4/paths.m4
    M m4/prog-ensure.m4
    M m4/sendfd.m4
    M m4/sendmail.m4
    M m4/syslog.m4
    M m4/users.m4

  Log Message:
  -----------
  Improve M4 style

- Homogenize the use of quotations with [ and ], and quadrigraphs.

- Use the AS_CASE, AS_ECHO, and AS_IF syntax.

- Replace define() with AC_DEFUN().