Add better support for Khronos C++ types

[kylelutz]

Add better support for interoperating between Boost.Compute and the Khronos OpenCL C++ wrapper types. For example, the following should be possible:

cl::CommandQueue q1 = ...;
boost::compute::command_queue q2 = q1;

[maddanio]

shouldn't it be easy to interoperate via the c-api level? i feel such wrappers should be quite trivial, but maybe i am wrong, maybe ownership is complicated?