Try to find the LibXml2 xml processing library

Once done this will define

LIBXML2_FOUND - System has LibXml2
LIBXML2_INCLUDE_DIR - The LibXml2 include directory
LIBXML2_LIBRARIES - The libraries needed to use LibXml2
LIBXML2_DEFINITIONS - Compiler switches required for using LibXml2
LIBXML2_XMLLINT_EXECUTABLE - The XML checking tool xmllint coming with LibXml2
LIBXML2_VERSION_STRING - the version of LibXml2 found (since CMake 2.8.8)