Update OpenCL C++ header to 1.2.4
Disable enqueueNativeKernelLaunch on 32bit windows

refs #24

This looks like a bug in OpenCL standard: ::clEnqueueNativeKernelLaunch accepts
user function with cdecl calling convention, and cl::enqueueNativeKernelLaunch
accepts stdcall function.

Disabling the function in our cl.hpp for now.