configure: raise minimum autoconf requirement to 2.70
Needed for builds on NetBSD to work correctly, since it depends on
AC_USE_SYSTEM_EXTENSIONS defining _OPENBSD_SOURCE to expose the
prototype for reallocarray() in the system headers.
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Needed for builds on NetBSD to work correctly, since it depends on
AC_USE_SYSTEM_EXTENSIONS defining _OPENBSD_SOURCE to expose the
prototype for reallocarray() in the system headers.