We need to follow the same specification as xmlrpc-c until we deprecate it. It is breaking various software such as sonarr. We can't have xmlrpc using i8 and tinyxml2 using i4, while we allow both to be used.
By default, builds will still not have XMLRPC enabled at all and the
configure flag `--with-xmlrpc-tinyxml2` must be specified. If both
xmlrpc-c and tinyxml2 are specified, xmlrpc-c takes precedence.
Basic benchmarks indicate tinyxml2 is 2x faster for small
requests/responses, and that only increases as response sizes get
larger.