Repeating another program's documentation is exactly the sort of problem I'm talking about: it creates an unnecessary maintenance burden.

Having re-read the other section about including getopt_long, I agree that it is best left there.

I am delighted to see that you use gnulib in gengetopt. However, I don't think it's a good idea to put detailed documentation on how to use gnulib in the gengetopt manual. The place for that is the gnulib manual. This is for two reasons: first, your  documentation duplicates the gnulib manual; and secondly, it risks getting out of date.

I suggest that you proceed as follows: in the section "Use automake/autoconf to check for the existence of `getopt_long'", remove the two sub-pages (there is no need to keep the old method "Use getopt_long sources"). Then, remove the main paragraph, and replace it with something like this:

If your program's build system uses the GNU autotools, the easiest way to ensure that you have `getopt_long' is to use gnulib's `getopt' module. See the gnulib manual, or http://www.gnu.org/s/gnulib/ for more information.

(By the way, I believe that the getopt-posix module should also work; if that's true you can change "`getopt'" to "`getopt' or
`getopt-posix'" above.)

If you think you have explained something better than the gnulib manual, then by all means submit a patch; I know the gnulib authors are always happy to receive patches to gnulib and its documentation.