Handle libxml2 2.9.8 API change.
Handle libxml2 2.9.8 API change.

libxml2 2.9.8 changes xmlHashScanner to take a xmlChar const * rather
than xmlChar *. We already changed our xmlHashScanner functions to take
a pointer to const xmlChar, but this breaks builds with older libxml2
versions. We now check the version and use the appropriate signature.

rug-compling/dact,rug-compling/dact