Creating the actual extension source is as easy as: 

$ udf-gen x1.xml

Creating 'x1' extension in '/home/hartmut/examples/x1'

Running 'aclocal'

Running 'autoconf'

Running 'automake'

Your extension has been created in directory x1.
See x1/README amd x1/INSTALL for further instructions.
This will create the following directory from x1.xml: 

x1
|-- AUTHORS
|-- COPYING 
|-- ChangeLog
|-- INSTALL
|-- Makefile.am
|-- Makefile.in
|-- NEWS
|-- README
|-- aclocal.m4
|-- autom4te.cache
|   |-- output.0
|   |-- requests
|   `-- traces.0
|-- config.guess 
|-- config.sub 
|-- configure
|-- configure.in
|-- depcomp 
|-- install-sh
|-- ltmain.sh 
|-- missing 
|-- mkinstalldirs
|-- udf_x1.h
`-- x1.cpp