libxml2 has traditionally been optional and only needed for XML parsing of stylesheets if the ptree parser available in boost is not used.
But include/mapnik/svg_parser.hpp now has a hard dependency on libxml2, which we should ideally try to make optional by adding support for ptree/rapidxml.
move bundled agg sources into depedencies subdirectory (as we may bun…
…dle other deps there too to reduce clutter) and remove all traces of unused tinyxml (ptree now uses rapidxml header-only library internally) - refs #913
ensure xml2 is present - punts on and closes #913
not going to move on this, not really that interested in using ptree in the svg_parser - no real benefit.
drop libxml now optional - refs mapnik/mapnik#913
Closed by #3003