gcc.c (option_map): Add --sysroot.

	* gcc.c (option_map): Add --sysroot.
	(process_command): Handle --sysroot.
	(display_help): Document it.
	* doc/cppopts.tex (-isysroot): Document.
	* doc/invoke.texi (--sysroot): Document.
	* doc/install.texi (--with-build-sysroot): Document.

	* Makefile.in (inhibit_libc): New variable.
	(INHIBIT_LIBC_CFLAGS): Likewise.
	(LIBGCC2_CFLAGS): Include
	$(INHIBIT_LIBC_CFLAGS).
	(CRTSTUFF_CFLAGS): Include $(INHIBIT_LIBC_CFLAGS).
	($(T)crtbegin.o): Do not use @inhibit_libc@.
	($(T)crtend.o): Likewise.
	($(T)crtbeginS.o): Do not use @inhibit_libc@.
	($(T)crtendS.o): Likewise.
	($(T)crtbeginT.o): Do not use @inhibit_libc@.
	($(T)crtendT.o): Likewise.
	(stmp-fixinc): Do not complain about missing headers if
	inhibit_libc.
	* configure.ac (inhibit_libc): Set it to true/false.
	(--with-build-sysroot): New option.  Use it to set
	SYSTEM_HEADER_DIR.
	* configure: Regenerated.

From-SVN: r102367