build: Update to newer autoconf constructs.
* configure.ac (AC_INIT): Use 'https://'. Use AC_CONFIG_HEADERS instead of AM_CONFIG_HEADER. Use AC_USE_SYSTEM_EXTENSIONS instead of AC_GNU_SOURCE. Use AS_HELP_STRING instead of AC_HELP_STRING. * gl/m4/onceonly_2_57.m4: Remove. * m4/gpg-error.m4: Update from libgpg-error. * m4/libtool.m4: Update from libgpg-error. * m4/libgcrypt.m4: Update from libgcrypt.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>