build: Update to new autoconf constructs.
* configure.ac Replace AC_GNU_SOURCE to Use AC_USE_SYSTEM_EXTENSIONS. Replace AC_HELP_STRING to AS_HELP_STRING. * m4/estream.m4: Replace AC_TRY_LINK to AC_LINK_IFELSE. * m4/gnupg-misc.m4: Replace AC_TRY_COMPILE to AC_COMPILE_IFELSE. * m4/libtool.m4: Use AC_LANG_PUSH/AC_LANG_POP instead of AC_LANG_SAVE/AC_LANG_RESTORE. * m4/readline.m4: Replace AC_HELP_STRING to AS_HELP_STRING. Fix quote for _combo. * m4/threadlib.m4: Replace AC_HELP_STRING to AS_HELP_STRING.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>