Compiler warnings about opencl-clhpp

[roy7]

I'm not sure if #34 might fix some of these already. :)

[jrprice]

Yes, the type qualifiers ignored on function return type warning should be fixed. The ignoring attributes on template argument warning is more problematic though - this isn't specific to the C++ bindings and happens with any C++ code that declares something like a std::vector<cl_int>.

What do you think about silencing these in this upstream code like Vc? VcDevel/Vc#164

[jrprice]

I've been debating this internally, but I think I'd be open to accepting a PR that uses those GCC pragmas to disable that warning for cl2.hpp.

[sethtroisi]

seems like this was fixed here in #47
and in LZ with leela-zero/leela-zero@c72cb3a

[roy7]

Thanks for the heads up @sethtroisi!