[boost] Boost.Thread fix for gcc 3.4

The test_thread test fails on g++3.4 on this line: BOOST_TEST(thrd == boost::thread()); saying that copy constructor of boost::noncopyable is not accessible. My understanding of the problem is that boost::thread() is rvalue. According to standard, implementation - bind reference to rvalue - create temporary the copy constructor, however, should be accessible no matter what implementation decides. Earlier gcc version did not enforce this rule, apparently. I've comitted the attached obvious fix, tested to work on 3.4, 3.3, 3.2 and 2.95. - Volodya