Vadim Zhukov authored Jul 11, 2017 and Brad King committed Jul 14, 2017

* 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.