- LIBXML2_INCLUDE_DIR talks about libxml2 itself;
- LIBXML2_INCLUDE_DIRS lists also include directories coming from libxml2's dependencies and is what libxml2 users should use;
- LIBXML2_LIBRARY talks about libxml2 itself;
- LIBXML2_LIBRARIES is what libxml2 users should link to.
This commit doesn't break existing users of FindLibXml2.cmake, but provides the way for correct behaviour in future. In particular, on OpenBSD LIBXML2_INCLUDE_DIR resolves to /usr/local/include/libxml2, while libiconv (an libxml2 dependency) puts iconv.h in /usr/local/include, which is not picked up automatically.