* io/Makefile (aux): Add have_o_cloexec.

* io/Makefile (aux): Add have_o_cloexec.

	* include/fcntl.h: Declare __have_o_cloexec.
	* io/have_o_cloexec.c: New file.
	* sysdeps/unix/opendir.c (__opendir): Use O_CLOEXEC is available.
	(__alloc_dir): If O_CLOEXEC has been used, don't duplicate the
	fcntl call if not necessary.
	* login/utmp_file.c (setutent_file): Use __have_o_cloexec instead
	of local variable.