Introduce extension Macros

Implement idea from KhronosGroup#248
to add an version macro to the extension headers so that
users can guard application code using the macro to ensure the correct
APIs are being used.

Extensions can then increment the version when they change the APIs
without breaking user code.

See
[CL_MAKE_VERSION](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#CL_MAKE_VERSION)
for how the macro version is defined.