From my pbuilder log, using dpkg-buildpackage -j2:

[SNIP]
  fakeroot debian/rules clean
[SNIP]
dh_testroot
Building lxml version 2.1.5.
NOTE: Trying to build without Cython, pre-generated 'src/lxml/lxml.etree.c' needs to be available.
dh_clean -k
Using build configuration of libxslt 1.1.24
Building against libxml2/libxslt in the following directory: /usr/lib
running install
running build
running build_py
running build_ext
running install_lib
creating /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
creating /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/_setmixin.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/soupparser.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/_diffcommand.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/_dictmixin.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/diff.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/builder.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/ElementSoup.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/defs.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/clean.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/__init__.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/formfill.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/html/usedoctest.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/html
copying build/lib.linux-x86_64-2.4/lxml/etree.so -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/_elementpath.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/cssselect.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/doctestcompare.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/builder.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/objectify.so -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/sax.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/__init__.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/ElementInclude.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/pyclasslookup.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
copying build/lib.linux-x86_64-2.4/lxml/usedoctest.py -> /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml
error: /tmp/buildd/lxml-2.1.5/debian/python-lxml/usr/lib/python2.4/site-packages/lxml/usedoctest.py: No such file or directory
make: *** [install-python2.4] Error 1
make: *** Waiting for unfinished jobs....