Resolution was to add the following line after line 26 to the file XPath2MemoryManager.hpp:
#include <cstddef>
The full path to the file is:
/usr/share/dbxml-2.5.16/xqilla/include/xqilla/framework/XPath2MemoryManager.hpp
Credit to http://tlmaloney.ruhoh.com/sysadmin/installing-bdb-xml-in-ubuntu/