Added cl_icdl.h header file.

[Kerilk]

This is a companion PR for:
KhronosGroup/OpenCL-ICD-Loader#188

[bashbaug]

I'm fine with as-is, but just to ask the question: should we consider putting these additions into the existing cl_ext.h file vs. adding a new file?

I guess these extensions are little different than the other extensions in cl_ext.h: at least currently they won't go into any queryable extension strings, for example. Perhaps that's a good enough reason to keep them separate?

Regardless, I think I'll try to update my header generation scripts to generate the defines and typedefs for this extension - seems like that should be pretty straightforward.

[Kerilk]

Thanks for the insightful feedback.

I'm fine with as-is, but just to ask the question: should we consider putting these additions into the existing cl_ext.h file vs. adding a new file?

Very good question. I discarded putting the extension in the cl_icd.h file as its usage is very specific and not application facing. I hadn't thought about cl_ext.h. Given users could want to invoke it from their applications it would make sense I guess. Tell me if you want me to go this route.

[Kerilk]

Closing as merged by #161