* configure.ac: Use AC_USE_SYSTEM_EXTENSIONS instead of AC_GNU_SOURCE.
Use AS_HELP_STRING instead of AC_HELP_STRING.
* m4/libtool.m4: Update from libgpg-error.
* m4/gpg-error.m4: Update from libgpg-error.
* m4/libassuan.m4: Update from libassuan.
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>