Policy for Extension Preprocessor Defines

[bashbaug]

The policy for extension preprocessor defines - meaning a #define for cl_my_extension_name in the headers - is currently a bit ad-hoc. It looks like the original intent was that an extension should have a define if it adds new API functions, but not otherwise, e.g.:

/* cl_khr_fp64 extension - no extension #define since it has no functions  */

But, some extensions include a define anyways, either because they erroneously included a define initially and this has been carried forward for backwards compatibility, or because an extension define can be useful even when it doesn't add new API functions - see #85 and https://github.com/KhronosGroup/OpenCL-CLHPP/pull/82.

Should we just add an extension define in all cases, uniformly?