Assume autoconf >= 2.64.

Assume autoconf >= 2.64.

* gnulib-tool (DEFAULT_AUTOCONF_MINVERSION): Set to 2.64.
* DEPENDENCIES: Require Autoconf 2.64 or newer.
* NEWS: Mention the changed requirement.
* m4/00gnulib.m4 (_m4_divert_diversion, AC_DEFUN_ONCE): Remove macros.
* m4/gnulib-common.m4 (AS_VAR_IF, AS_VAR_COPY): Remove macros.
* m4/absolute-header.m4 (gl_ABSOLUTE_HEADER): Quote pushdef'ed
variables.
* m4/include_next.m4 (gl_NEXT_HEADERS_INTERNAL): Likewise.
* m4/warn-on-use.m4 (gl_WARN_ON_USE_PREPARE): Likewise.
* m4/warnings.m4 (gl_COMPILER_OPTION_IF): Likewise.
(gl_UNKNOWN_WARNINGS_ARE_ERRORS(C), gl_UNKNOWN_WARNINGS_ARE_ERRORS(C++),
gl_UNKNOWN_WARNINGS_ARE_ERRORS(Objective C)): Define through AC_DEFUN.
* m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC(C), gl_MANYWARN_ALL_GCC(C++)):
Likewise.
* m4/iconv.m4 (AM_ICONV): Define through AC_DEFUN_ONCE directly.
* m4/libunistring.m4 (gl_LIBUNISTRING): Likewise.
* m4/configmake.m4: Update comment.