xorg-sgml-doctools: merge request

[tlaronde at kergis.com]

CI passed.

I have modified meson.build and added a meson_options.txt so that
with meson it is possible to set install directory with the
flexibility offered by the autotools.

Namely, autotools allows to set datadir and datarootdir. datarootdir
is for data added; datadir is used to place pkgconfig pc files.

Furthermore (and this is partly an extension), I have added a meson
option: pkgconfigdir to mimick the (unavailable) autotools
--with-pkgconfigdir to se the directory where the pc files have to be
put.

There is one thing that will have to be decided globally.

For now, pkgconfig files are installed in ${datadir}/pkgconfig.

But "normally", pkgconfig files have to be installed in
${libdir}/pkgconfig meaning that Xorg pc files can end elsewhere on
systems than other pkgconfig files (and this is a bit contrary to the
purpose of the pkgconfig files: a well-known place to get information
about where is the stuff and how to link etc.).

Caveats for others: I had to add the meson_options.txt to
Makefile.am:EXTRA_DIST, since, if not, the file was not packaged.

-- 
        Thierry Laronde <tlaronde +AT+ kergis +dot+ com>
                     http://www.kergis.com/
                    http://kertex.kergis.com/
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