Implement joining_thread

[kuzkry]

Note that this is rebased on top of #807 but it consists of one commit only (therefore I suggest going to the commit section and simplifying your review this way).

This PR implements a simple version of joining_thread. Appropriate thread library is detected and supplied by CMake.
Reasons why joining_thread derives privately from std::thread:

• basically private inheritance works like composition with different semantics
• it allows to get bunch of usings instead of redefining the whole std::thread interface
• no problems with undefined behaviour if a user would try to delete joining_thread through the base class pointer
• prevention of doing tricks, such as:

    joining_thread a([]{});
    a.detach() // compile error, detach() is deleted
    a.std::thread::detach(); // no error, though a loophole

Last but not least, I didn't care much about exceptions thrown by joinable(), I assumed it would have to be an incredibly crazy scenario for this function to throw, but noexcept(false) could theoretically be added there. I say "theoretically", because the same function is called by the destructor of std::thread anyway which would terminate in this case.

[kuzkry]

My bad, I didn't notice another pull request that tries to merge the same thing.