maint: use modules/tempname.diff file, not a replacement; update gnulib

maint: use modules/tempname.diff file, not a replacement; update gnulib

Using a .diff is much more maintainable.  Otherwise, changes in
the gnulib module description file may not be noticed and merged
promptly and may even result in subtle errors.  Luckily, this time,
the failure to propagate gnulib's changes to modules/tempname resulted
only in an obvious link failure.
* gl/modules/tempname: Remove file.
* gl/modules/tempname.diff: Use a .diff file instead.
* gnulib: Update submodule to latest.