Fixing zlib dependency for libxml2.

vtklibxml2 depends on zlib but was never setting up a dependency for it nor was
it using the right variable after modularization changes. Fixed that.

Change-Id: I5932313ea5b2cdba47e885a728778abc5d6739f8
Merge topic 'fix_vtklibxml2_zlib_dependency' into master

d6a49b93 Fixing zlib dependency for libxml2.