Ensure XML entities are loaded when building docs

Recent versions of libxml2 disable entity loading by default. This change calls
`libxml_disable_entity_loader(false)`, and also passes the `LIBXML_NOENT` flag
to each instance of `DOMDocument::load()` to ensure that entities may be loaded.

This fixes a build error whereby `xi:include` and related tags are not
recognized due to the entities being loaded from a file instead of defined
inline.