Only test NULL and 0 arguments which is the only useful test from my
experience.
Use ElementTree instead of libxml2's own Python bindings in gentest.py.
Stop using the <cond> element in libxml2-api.xml. Look up modules
required by symbols using separate code in xmlmod.py, preparing for the
eventual removal of apibuild.py.
Support for RELAX NG used to be enabled together with XML Schema support
(--with-schemas). Now there's a separate option and a new feature macro
LIBXML_RELAXNG_ENABLED.
