Implementing accessibility non-regression check tool

[Stephan Bergmann]

I'm not sure basing this gla11y tool on lxml (<http://lxml.de/>) is a 
good choice:  "The lxml XML toolkit is a Pythonic binding for the C 
libraries libxml2 and libxslt." (<http://lxml.de/>)

Different LO builds have the choice/need to use either a system Python 
or a locally-built one from external/python3.

Different LO builds have the choice/need to use either a system 
libxml2/libxslt or a locally-built one from external/{libxml2,libxslt}.

Adding lxml into the mix, different LO builds will have the choice/need 
to use either a system lxml or a locally-built one.  (Which 
<https://gerrit.libreoffice.org/#/c/50115/3> "Build external lxml if not 
provided by system" is going to add as external/lxml.)

That means external/lxml will need to:

* build against and run with either the system Python or the 
locally-built one from external/python3,

* build against and run with either the system libxml2/libxslt or the 
locally-built one from external/{libxml2,libxslt}.

That's four different ways how to build external/lxml, and slightly more 
different ways (using the system lxml versus external/lxml) how to run 
the gla11y tool in solenv/gbuild/UIConfig.mk.  And getting all those 
ways to work, on the different platforms, will be hell.

Isn't there another option to make that gla11y tool process XML data, 
one that better matches LO's needs?