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