Cleanup
[external] Remove tinyxml/tinystr.cpp from built sources

Since we build TinyXML with -DTIXML_USE_STL this file is actually empty
and will just cause a "has no symbols" warning on OSX.
[YARP_OS] Mark variables as unused
[wire_rep_utils] Call yarp::os::exit instead of exit
[wire_rep_utils] Fix -Wsign-compare warnings
[YARP_OS] SO_LINGER is a SOL_SOCKET option