build: Update configure.ac for newer autoconf.
* configure.ac (AC_PREREQ): Require 2.69 or later. (AC_CONFIG_HEADERS): Replace AC_CONFIG_HEADER. (AC_HEADER_STDC): Remove. * m4/codeset.m4: Update from gnulib.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>