libstdc++: Filter out unconditional <stdio.h> default include

libstdc++: Filter out unconditional <stdio.h> default include

_AC_INCLUDES_DEFAULT_REQUIREMENTS including <stdio.h> when checking for
stdint.h has prevented proper detection of whether stdint.h is present,
since it'd poison the cache variables with test results failing due to
<stdio.h> failing to include. As a solution, for autoconf versions under
2.70, we filter out that bit of code from ac_includes_default.

This issue was fixed in autoconf-2.70.
This also applies to various other headers, but was noticed when looking
into why HAVE_STDINT_H was misdefined.

libstdc++-v3/ChangeLog:

	* configure.ac: Remove any lines that unconditionally include
	<stdio.h> from ac_includes_default, when running Autoconf <2.70.
	* configure: Regenerate.