Re-introduce linking to boost::thread.
The asio library needs to link to boost::thread, at least on OS X.
RippleSSLContextImp:  Uninline large member funcs
ScopedDHPointer:  Inhibit copies and be a class

Disable the copy constructor and copy assignment operator.  (RAII's rule
of three applies.)

TODO:  Replace this with std::unique_ptr<> (unless we remove DH first).
Extract ScopedDHPointer from RippleSSLContextImp