Fix CMake Error: xmlrpcvalue_base64 not built by.. (#1197)

Error occurs when using catkin-tools or catkin_make_isolated --install.
catkin_make_isolated didn't complain without the --install option.

CMake Error at .../xmlrpcpp/test/CMakeLists.txt:2 (target_link_libraries):
  Cannot specify link libraries for target "xmlrpcvalue_base64" which is not
  built by this project.

utilities/xmlrpcpp/test/CMakeLists.txt

@@ -1,2 +1,4 @@
 catkin_add_gtest(xmlrpcvalue_base64 xmlrpcvalue_base64.cpp)
-target_link_libraries(xmlrpcvalue_base64 xmlrpcpp)
+if(TARGET xmlrpcvalue_base64)
+  target_link_libraries(xmlrpcvalue_base64 xmlrpcpp)
+endif()