Defining `CL_TARGET_OPENCL_VERSION`

[coatless]

As I am preparing a PR, what shall we do about the

-PKG_CPPFLAGS = -I../inst/include
+PKG_CPPFLAGS = -I../inst/include -DCL_TARGET_OPENCL_VERSION=300

which I need to not have OpenCL nag all the time. Any other systems / OSs have that?

Originally posted by @eddelbuettel in #6

See also https://gitlab.com/bandicoot-lib/bandicoot-code/-/issues/1 , it has been an outstanding issue for a very long time and I don't have a great solution for it. The situation is this:

1. OpenCL warns if CL_TARGET_OPENCL_VERSION is not set when it is included;
2. OpenCL will also warn if a CL_TARGET_OPENCL_VERSION is set that is newer than whatever the OpenCL distribution on the system is;
3. We aren't guaranteed really anything about what OpenCL version a user is using;
4. Bandicoot does its best to support whatever OpenCL version is available, so it does not set CL_TARGET_OPENCL_VERSION (which will restrict functionality to whatever that version is);

and OpenCL appears to provide no way to disable the warning or set some macro such that whatever the OpenCL header version is is what is used.

If from the R side you are able to say "we will always have OpenCL 3.0+" then you can just define it as you suggested above and be good with it, but I can't really do that upstream.

Originally posted by @rcurtin in #6

[coatless]

Presently, we're trying to use the version of OpenCL available with Xcode CLI. This means we have OpenCL v1.2 -- released on November 14, 2012 -- due to Apple deprecating it in favor of Metal.

https://developer.apple.com/opencl/

OpenCL v1.2 is the minimal version allowable by Bandicoot. Though, it's now over 13 years old, so I'm happy to move toward newer headers released by khronos.org/registry/OpenCL/

https://formulae.brew.sh/formula/opencl-headers

[coatless]

Closed by #11