PR libstdc++/33203
	* configure.host (error_constants_dir):  New variable,
	defaulting to os/generic.
	( mingw32*): Overide default.
	* configure.ac (ERROR_CONSTANTS_SRCDIR): New.
	* configure: Regenerate.
	* include/Makefile.am (host_headers): Add
	$(ERROR_CONSTANTS_SRCDIR)/error_constants.h .
	* include/Makefile.in: Regenerate.
	* include/std/system_error (enum posix_errno): Remove.
	Include <bits/error_constants.h> instead.
	* config/os/mingw32/error_constants.h: New.
	* config/os/generic/error_constants.h: New.

From-SVN: r128228