Use auto_ptr when compiling with pre C++11 compilers

[HerbertKoelman]

This should avoid this message:

thread.cpp:126:12: warning: ‘template<class> class std::auto_ptr’ is deprecated [-Wdeprecated-declarations]
       std::auto_ptr<pthread::abstract_thread> pat(_threads.front());
            ^
In file included from /usr/include/c++/5.3.1/memory:81:0,
                 from ../include/pthread/thread.hpp:15,
                 from thread.cpp:9:
/usr/include/c++/5.3.1/bits/unique_ptr.h:49:28: note: declared here
   template<typename> class auto_ptr;