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/rpc/xmlrpc_test.h

@@ -0,0 +1,28 @@
+#include <cppunit/extensions/HelperMacros.h>
+
+#include "rpc/command_map.h"
+#include "rpc/xmlrpc.h"
+
+class XmlrpcTest : public CppUnit::TestFixture {
+  CPPUNIT_TEST_SUITE(XmlrpcTest);
+  CPPUNIT_TEST(test_basics);
+  CPPUNIT_TEST(test_invalid_utf8);
+  CPPUNIT_TEST_SUITE_END();
+
+public:
+  static const int cmd_size = 256;
+
+  void setUp();
+  void tearDown() {}
+
+  void test_basics();
+  void test_invalid_utf8();
+
+private:
+  rpc::XmlRpc m_xmlrpc;
+  
+  rpc::CommandMap m_map;
+
+  rpc::command_base m_commands[cmd_size];
+  rpc::command_base* m_commandItr;
+};