There used to be the configure option `--with-full-gnats' but that was
removed. However, gnats/Makefile.in still contains the *-gnats and
*-tools targets serving exactly that purpose. They only don't seem to
be propagated to the top-level Makefile.
BTW, the top-level Makefile looks like a horrible mess. It's three
times larger than the Emacs one and it seems to be more general than
needed, I don't like it. Unless anyone objects, I'll clean it up,
together with moving the documentation to a separate directory.