Mir FTBFS with gcc 4.9.1-14 (utopic update): auto_unblock_thread.h:44:46: error: no matching function for call to ‘std::thread::thread(<brace-enclosed initializer list>)’

Since updating my utopic build machine today, Mir no longer builds:

In file included from /home/dan/bzr/mir/dev/tests/unit-tests/compositor/test_buffer_queue.cpp:26:0:
/home/dan/bzr/mir/dev/tests/include/mir_test/auto_unblock_thread.h: In instantiation of ‘mir::test::AutoUnblockThread::AutoUnblockThread(const std::function<void()>&, Callable&&, Args&& ...) [with Callable = void (&)(mir::compositor::BufferQueue&, std::atomic<bool>&); Args = {std::reference_wrapper<mir::compositor::BufferQueue>, std::reference_wrapper<std::atomic<bool> >}]’:
/home/dan/bzr/mir/dev/tests/unit-tests/compositor/test_buffer_queue.cpp:426:59: required from here
/home/dan/bzr/mir/dev/tests/include/mir_test/auto_unblock_thread.h:44:46: error: no matching function for call to ‘std::thread::thread(<brace-enclosed initializer list>)’
         : unblock{unblock}, thread{f, args...}
                                              ^
/home/dan/bzr/mir/dev/tests/include/mir_test/auto_unblock_thread.h:44:46: note: candidates are:
In file included from /home/dan/bzr/mir/dev/tests/include/mir_test/auto_unblock_thread.h:29:0,
                 from /home/dan/bzr/mir/dev/tests/unit-tests/compositor/test_buffer_queue.cpp:26:
/usr/include/c++/4.9/thread:133:7: note: std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(mir::compositor::BufferQueue&, std::atomic<bool>&); _Args = {std::reference_wrapper<mir::compositor::BufferQueue>&&&, std::reference_wrapper<std::atomic<bool> >&&&}]
       thread(_Callable&& __f, _Args&&... __args)
       ^
/usr/include/c++/4.9/thread:133:7: note: no known conversion for argument 2 from ‘std::reference_wrapper<mir::compositor::BufferQueue>&&’ to ‘std::reference_wrapper<mir::compositor::BufferQueue>&&&’
/usr/include/c++/4.9/thread:128:5: note: std::thread::thread(std::thread&&)
     thread(thread&& __t) noexcept
     ^
/usr/include/c++/4.9/thread:128:5: note: candidate expects 1 argument, 3 provided
/usr/include/c++/4.9/thread:122:5: note: std::thread::thread()
     thread() noexcept = default;
     ^

....