building lxml requires additional configuration so it finds xslt-config correctly. One possibility is to set the PATH environment variable. The other is to call setup.py --xslt-config=...
Using setup.py --xslt-config doesn't fit at all into buildout i think.
Setting environment variables before installing an egg on the other hand might be useful. Would it make sense to put this into zc.recipe.egg or would it be better to create some unrelated recipe which only sets envirionment variables?