Check That Atomics Are Trivially Initializable In Configure

[insertinterestingnamehere]

While fixing #186, I noticed that there's an implicit assumption that atomics are trivially initializable. This is fine since it's true on all the architectures we currently support, but in the future we should either check that this is true or remove the assumption. The easiest way is probably just to assert somewhere in the configure script that (in C++11) static_assert(std::is_trivially_constructible<std::atomic<std::uintptr_t>>::value, "Trivially initializable atomics are required.")