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.

src/Makefile.am

@@ -121,6 +121,8 @@ libsub_root_a_SOURCES = \
 	rpc/scgi_task.h \
 	rpc/xmlrpc.h \
 	rpc/xmlrpc.cc \
+	rpc/xmlrpc_c.cc \
+	rpc/xmlrpc_tinyxml2.cc \
 	\
 	ui/download.cc \
 	ui/download.h \
@@ -188,6 +190,10 @@ libsub_root_a_SOURCES = \
 	thread_base.cc \
 	thread_base.h \
 	thread_worker.cc \
-	thread_worker.h
+	thread_worker.h \
+	\
+	vendor/tinyxml2/tinyxml2.h \
+	vendor/tinyxml2/tinyxml2.cpp
+
 
 AM_CPPFLAGS = -I$(srcdir) -I$(top_srcdir)