copied over the apibuild.py from libxml2, adapted a bit and regenerated

* doc/Makefile.am doc/apibuild.py doc/libxslt-api.xml:
  copied over the apibuild.py from libxml2, adapted a bit and
  regenerated the API description in XML. Todo: libexslt-api.xml
* libxslt/attributes.c libxslt/documents.c libxslt/extensions.c
  libxslt/imports.c libxslt/numbers.c libxslt/numbersInternals.h
  libxslt/pattern.c libxslt/preproc.c libxslt/security.c
  libxslt/templates.c libxslt/transform.c libxslt/transform.h
  libxslt/variables.c libxslt/xslt.c libxslt/xsltInternals.h
  libxslt/xsltutils.c libxslt/xsltutils.h: cleanup based on the
  report from the scripts.
* libxslt.spec.in: make sure libxslt-api.xml ends up in the devel
  package
Daniel