XmlRpcpp fixes for OSX

[trainman419]

Fix compilation and polling issues in XmlRpcpp on OSX. This fixes some but not all of the issues reported in #1357

[Vibrane]

All patches work.

[dirk-thomas]

The CI build for Melodic shows two compiler warnings:

In file included from /tmp/catkin_workspace/src/ros_comm/utilities/xmlrpcpp/test/mock_socket.h:26:0,
                 from /tmp/catkin_workspace/src/ros_comm/utilities/xmlrpcpp/test/mock_socket.cpp:26:
/usr/src/googletest/googletest/include/gtest/gtest.h: In instantiation of ‘testing::AssertionResult testing::internal::CmpHelperLE(const char*, const char*, const T1&, const T2&) [with T1 = int; T2 = long unsigned int]’:
/tmp/catkin_workspace/src/ros_comm/utilities/xmlrpcpp/test/mock_socket.cpp:63:3:   required from here
/usr/src/googletest/googletest/include/gtest/gtest.h:1524:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
/usr/src/googletest/googletest/include/gtest/gtest.h:1510:7:
   if (val1 op val2) {\
       ~~~~~~~~~             
/usr/src/googletest/googletest/include/gtest/gtest.h:1524:28:
 GTEST_IMPL_CMP_HELPER_(LE, <=);
                             
/usr/src/googletest/googletest/include/gtest/gtest.h:1510:12: note: in definition of macro ‘GTEST_IMPL_CMP_HELPER_’
   if (val1 op val2) {\
            ^~
/usr/src/googletest/googletest/include/gtest/gtest.h: In instantiation of ‘testing::AssertionResult testing::internal::CmpHelperEQ(const char*, const char*, const T1&, const T2&) [with T1 = int; T2 = long unsigned int]’:
/usr/src/googletest/googletest/include/gtest/gtest.h:1459:23:   required from ‘static testing::AssertionResult testing::internal::EqHelper<true>::Compare(const char*, const char*, const T1&, const T2&, typename testing::internal::EnableIf<(! testing::internal::is_pointer<T2>::value)>::type*) [with T1 = int; T2 = long unsigned int; typename testing::internal::EnableIf<(! testing::internal::is_pointer<T2>::value)>::type = void]’
/tmp/catkin_workspace/src/ros_comm/utilities/xmlrpcpp/test/mock_socket.cpp:302:3:   required from here
/usr/src/googletest/googletest/include/gtest/gtest.h:1392:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (lhs == rhs) {
       ~~~~^~~~~~

Can you please address those as well in order to merge this. Thanks.

[dirk-thomas]

I addressed the compiler warnings in #1402 in order to merge this. Closing in favor of that PR.

@trainman419 Thank you for the patch.