Add support for CL_HPP_OPENCL_API_WRAPPER macro (#314)

Add support for a macro CL_HPP_OPENCL_API_WRAPPER which will be used to
wrap all OpenCL API calls.This can be used e.g. to load the OpenCL
library dynamically with dlopen()/dlsym().

Co-authored-by: Ben Ashbaugh <[email protected]>