threading.hxx: correctly deal with GCC 4.6/4.7

This "fixes" compilation with GCC 4.6 / 4.7 which were not configured
with --enable-libstdcxx-time, thus not having std::this_thread::yield().
(Actually, vigra will still not compile, but it will properly point you
to -DWITH_BOOST_THREAD=1.)  See GH issue #220.