* include/cpplib.h (struct cpp_options): Add new boolean flag
	warn_builtin_macro_redefined.
	* init.c (cpp_create_reader): Initialize warn_builtin_macro_redefined.
	* (struct builtin_operator): Split out from previous struct builtin,
	enhance extra const correctness.
	* (struct builtin_macro): Split out from previous struct builtin, add
	new always_warn_if_redefined flag, enhance const correctness.
	* (mark_named_operators): Use struct builtin_operator.
	* (cpp_init_special_builtins): Use struct builtin_macro, add NODE_WARN
	to builtins selectively.
	* macro.c (warn_of_redefinition): Return false if a builtin macro
	is not flagged with NODE_WARN.

	* c-opts.c (c_common_handle_option): Add handling for
	-Wbuiltin-macro-redefined command line option.
	* c.opt: Added builtin-macro-redefined option.
	* doc/invoke.texi (Warning Options): Add -Wbuiltin-macro-redefined
	documentation.

	* gcc.dg/builtin-redefine.c: New.

From-SVN: r140461