Get rid of the dependency on sgml2latex, which comes from
        a huge, obsolete, non-portable and broken package.

        Use reStructuredText from Docutils instead.
        See: http://docutils.sourceforge.net/rst.html

        * doc/xml/dev.sgml: Rewrite to...
        * doc/xml/dev.txt: ...this. Fix (some) mistakes.
        * doc/xml/format.sgml: Rewrite to...
        * doc/xml/format.txt: ...this. Fix (some) mistakes.

        * doc/xml/Makefile.am: Update accordingly. Cleanup.

        * configure.ac: Configure for RST2TEX. Remove configuration for
        SGML2TEX.

        * doc/beginner/Makefile.am,
        * doc/tutorial/Makefile.am: Use properly autoconf expansions.