* c.opt (Wmissing-include-dirs): New.
	* c-opts.c (c_common_handle_option): Pass true for user_supplied_p
	to add_path () for -I, but false for OPT_idirafter, OPT_iquote and
	OPT_isystem. Handle case OPT_Wmissing_include_dirs.
	* c-incpath.h (add_path): Add fourth (bool) argument.
	* c-incpath.c (add_env_var_paths): Pass false to add_path ().
	(add_standard_paths): Likewise.
	(remove_duplicates) [REASON_NOENT]: Warn if -Wmissing-include-dirs
	is used and the directory was user-supplied via -I.
	(add_path): Set p->user_supplied_p.  Remove duplicated code by
	using add_cpp_dir_path ().
	* cpplib.h (struct cpp_options): Add warn_missing_include_dirs.
	(struct cpp_dir): Add user_supplied_p.
	* doc/invoke.texi (Warning Options): Document new option.

[testsuite]
	* gcc.dg/cpp/Wmissingdirs.c: New.

From-SVN: r82121