gnutls does not depend on autogen for the generation of manpages/texinfo documentation

This is about avoiding to use of autogen for the generation of the manpages and texinfo documentation of the tools in src/. The tricky part is that the use of autogen eliminates the risk of out-of-date documentation of these tools, as the documentation is auto-generated from the definitions. Any replacement should not re-introduce that risk as past experience showed that documentation of tools was quickly getting our of sync with the tool options.