configure.ac: Add --enable-werror-always (for top level bootstrap support).

configure.ac: Add --enable-werror-always (for top level bootstrap support).

(gcc)
	* configure.ac: Add --enable-werror-always (for top level bootstrap
	support).
	* configure: Regenerate.

(top level)
	* configure.in (top level bootstrap support): Rework --enable-werror
	to set @stage2_werror_flag@.
	* configure: Regenerate.
	* Makefile.tpl (top level bootstrap support): Pass
	@stage2_werror_flag@ down to configure in stages 2 and 3.
	* Makefile.in: Regenerate.

From-SVN: r79935