Allow using vendored tinyxml2 for XMLRPC

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.

test/src/command_dynamic_test.cc

@@ -17,7 +17,7 @@ void initialize_command_ui();
 
 void
 CommandDynamicTest::setUp() {
-  if (rpc::commands.empty()) {
+  if (rpc::commands.find("method.insert") == rpc::commands.end()) {
     setlocale(LC_ALL, "");
     cachedTime = rak::timer::current();
     control = new Control;