* configure.ac: Add --with-native-system-header-dir.  Set and
	substitute NATIVE_SYSTEM_HEADER_DIR.  Use native_system_header
	when setting target_header_dir.
	* config.gcc: Always set native_system_header_dir.
	(*-*-gnu*): Set native_system_header_dir.  Don't use t-gnu.
	(i[34567]86-pc-msdosdjgpp*): Set native_system_header_dir.  Don't
	use i386/t-djgpp.
	(i[34567]86-*-mingw* | x86_64-*-mingw*): Set
	native_system_header_dir.
	(spu-*-elf*): Set native_system_header_dir.
	* Makefile.in (NATIVE_SYSTEM_HEADER_DIR): Set to
	@NATIVE_SYSTEM_HEADER_DIR@.
	(PREPROCESSOR_DEFINES): Define NATIVE_SYSTEM_HEADER_DIR.
	* cppdefault.c (STANDARD_INCLUDE_DIR): Don't define.
	(NATIVE_SYSTEM_HEADER_COMPONENT): Rename from
	STANDARD_INCLUDE_COMPONENT.
	(cpp_include_defaults): Don't use SYSTEM_INCLUDE_DIR.  Rename
	STANDARD_INCLUDE_DIR to NATIVE_SYSTEM_HEADER_DIR.
	* system.h: Poison SYSTEM_INCLUDE_DIR, STANDARD_INCLUDE_DIR, and
	STANDARD_INCLUDE_COMPONENT.
	* config/i386/t-mingw32 (NATIVE_SYSTEM_HEADER_DIR): Remove.
	* config/i386/t-mingw-w32: Likewise.
	* config/i386/t-mingw-w64: Likewise.
	* config/spu/t-spu-elf: Likewise.
	* config/i386/t-djgpp: Remove.
	* config/t-gnu: Remove.
	* config/i386/mingw32.h (STANDARD_INCLUDE_DIR): Don't define.
	(NATIVE_SYSTEM_HEADER_COMPONENT): Rename from
	STANDARD_INCLUDE_COMPONENT.
	* config/i386/djgpp.h (STANDARD_INCLUDE_DIR): Don't define.
	* config/spu/spu-elf.h: Likewise.
	* config/vms/xm-vms.h: Likewise.
	* config/gnu.h: Likewise.
	* config/openbsd.h (INCLUDE_DEFAULTS): Change STANDARD_INCLUDE_DIR
	and STANDARD_INCLUDE_COMPONENT to NATIVE_SYSTEM_HEADER_DIR and
	NATIVE_SYSTME_HEADER_COMPONENT.
	* doc/install.texi (Configuration): Document
	--with-native-system-header-dir.  Mention it in the documentation
	for --with-sysroot and --with-build-sysroot.
	* doc/tm.texi.in (Driver): Don't document SYSTEM_INCLUDE_DIR or
	STANDARD_INCLUDE_DIR.  Rename STANDARD_INCLUDE_COMPONENT to
	NATIVE_SYSTEM_HEADER_COMPONENT.  Rename uses of
	STANDARD_INCLUDE_DIR to NATIVE_SYSTEM_HEADER_DIR.
	* doc/fragments.texi (Target Fragment): Don't document
	NATIVE_SYSTEM_HEADER_DIR.
	* configure, doc/tm.texi: Rebuild.

Co-Authored-By: Ian Lance Taylor <iant@google.com>

From-SVN: r180114