Fixing several warnings given by pedantic g++-4.8.2

This fixes several warnings (mostly unused local typedefs, and one unused parameter) given by g++-4.8.2 with `-pedantic` flag. The one with unused parameter is interesting, because it is in a copy constructor, and the [comment says](https://github.com/ddemidov/compute/blob/6c8f158c004238d261bab9b6494c60cb900f61da/include/boost/compute/wait_list.hpp#L35-L38) the parameter would be copied, but the constructor is empty.

I've seen the warnings when compiling Boost.Compute wrapping code for VexCL, which only uses scan and sort algorithms, so there could be more.