[boost] [smart_ptr][thread][test] breaking change on shared_ptr operator bool.

Hi, https://svn.boost.org/trac/boost/changeset/81780 " Use explicit operator bool when available; ..." introduces a breaking change on shared_ptr in Boost.Thread and Boost.Test at least. I would preferred if this breaking change was announced and why not added conditionally so that the user could move smoothly to the new semantic. Anyway, I wanted to make this evident so that the Boost maintainer using this facility fixes their code as soon as possible. Best, Vicente