Switch from recursive to non-recursive use of Gnulib.

This allows for better use of dependencies in Makefiles.  It causes a new
warning from Automake, which I hope to fix through a change to Gnulib
sometime soon.  I'm consulting the other Gnulib maintainers on that:
https://lists.gnu.org/archive/html/bug-gnulib/2022-03/msg00044.html