don't move return value
don't move return value

- `T f() { T result; ...; return result; }` works with movable (but
  noncopyable) types if C++11 is supported correctly; std::move prevents
  proper return value optimization. use BOOST_MOVE_RET if needed for
  compatibility with broken compilers.

- fixes ahupowerdns/luawrapper#4

ahupowerdns/luawrapper,ahupowerdns/luawrapper