build: Update m4/iconv.m4.

Authored by gniibe on May 13 2019, 8:19 AM.

Description

build: Update m4/iconv.m4.

* m4/iconv.m4: Update from gettext 0.20.1.

This includes fixes of file descriptor leaks.

  • GnuPG-bug-id: T4504
  • Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>