Disable option checking in the toplevel, avoid 2.64 fopen glitch.

Disable option checking in the toplevel, avoid 2.64 fopen glitch.

/:
	* configure.ac: Call AC_DISABLE_OPTION_CHECKING.
	(baseargs): Add --disable-option-checking.
	* configure: Regenerate.

config/:
	* override.m4 (AC_DISABLE_OPTION_CHECKING): Define to be
	empty if not defined, to avoid error with 2.59.
	(_AC_LANG_IO_PROGRAM): When the Autoconf version is exactly
	2.64, avoid per-language instances to drop fopen from test
	program.