[dovecot/core] 9c257a: m4: Fix compiler warnings

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 9c257ae400f8a075946a7457b45b2ff0a455ee21
      https://github.com/dovecot/core/commit/9c257ae400f8a075946a7457b45b2ff0a455ee21
  Author: Aki Tuomi <aki.tuomi at dovecot.fi>
  Date:   2016-11-07 (Mon, 07 Nov 2016)

  Changed paths:
    M m4/fd_passing.m4
    M m4/ipv6.m4
    M m4/unsetenv.m4

  Log Message:
  -----------
  m4: Fix compiler warnings

Makes it work with -Wall -W -Werror