* lib/dirent_.h: Prefer #include_next <foo.h> to #include

* lib/dirent_.h: Prefer #include_next <foo.h> to #include
@ABSOLUTE_FOO_H@ if @HAVE_INCLUDE_NEXT@.  This works better with
GCC 4.2, which otherwise issues a lot of warnings.
* lib/iconv_.h, lib/locale_.h, lib/netinet_in_.h, lib/sys_select_.h:
* lib/sys_socket_.h, lib/sys_stat_.h, lib/sysexits_.h, lib/unistd_.h:
Likewise.
* modules/fchdir (dirent.h): Substitute @HAVE_INCLUDE_NEXT@.
* modules/iconv_open (iconv.h): Likewise.
* modules/locale (locale.h): Likewise.
* modules/netinet_in (netinet/in.h): Likewise.
* modules/sys_select (sys_select.h): Likewise.
* modules/sys_socket (sys/socket.h): Likewise.
* modules/sys_stat (sys/stat.h): Likewise.
* modules/sysexits (sysexits.h): Likewise.
* modules/unistd (unistd.h): Likewise.