Add lxml + fix libxml2 and libxslt builds.
Merge branch 'master' into lxml-lw

Conflicts:
	pkgs/libxslt.yaml
lxml: Roll back to 3.2.5 which works with cython 0.19.2
Depend on zlib when building libxslt
Merge pull request #472 from jcftang/lxml-lw

Lxml from https://github.com/hashdist/hashstack/pull/280