Fix compilation of #146 without MPI
Merge pull request #159 from kronbichler/master

Fix compilation of #146 without MPI. This closes #158.
Add Threads::Task::joinable().

The function verifies that one can call Threads::Task::join() in the
same way as one can do with std::thread::joinable() in C++11.
Add test.
Merge pull request #157 from bangerth/add-task-joinable

Add Threads::Task::joinable()