Add support for building documents in our doc/ tree marked up in
Norman Walsh's DocBook Slides DTD.

This DTD offers the vocabulary of simplified DocBook for
presentations. Initially, the supported output formats are PDF and
HTML.

XSL stylesheets are used so libxslt is required.

PassiveTeX is used for the PDF generation to convert the XSL-FO
directly to PDF.

This commit moves various TeX definitions out of doc.docbook.mk and
into doc.project.mk, since docbook is no longer the only back-end to
utilize TeX.

An example Makefile would look like :

DOCFORMAT= slides
DOC= slides

SRCS= slides.xml

DOC_PREFIX?= ${.CURDIR}/../../..

.include "${DOC_PREFIX}/share/mk/doc.project.mk"

And an example slides file (slides.xml) looks like :

<!DOCTYPE slides PUBLIC "-Norman WalshDTD Slides XML V3.3.1//EN"

"http://docbook.sourceforge.net/releases/slides/3.3.1/slides.dtd">

<slides>
<slidesinfo>

<title>What's new in FreeBSD 5.3</title>
<titleabbrev>FreeBSD 5.3</titleabbrev>

</slidesinfo>

<foil><title>Introduction Slide</title>

<para>Content</para>

</foil>

</slides>

You could then build the HTML and PDF versions of the slides by typing
"make FORMATS='pdf html'". Enjoy.

Please coordinate with doceng@ before importing any presentations to
doc/.