libstdc++: Deprecate the --enable-cheaders=c_std configuration

These headers do not offer any tangible benefit compared to the default
c_global version. They are not actively maintained meaning that they
have bugs which have already been fixed for the c_global headers.

This change adds a warning if they are used, and requires a new
--enable-cheaders-obsolete option to allow their use. Unless we receive
reports from users who rely on the c_std headers they should be removed
at some point in future.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_CHEADERS): Warn if the c_std
	option is used and fail unless --enable-cheaders-obsolete is
	also used.
	* configure: Regenerate.