Reconsider --enable-opencl / --disable-opencl

[solardiz]

We no longer depend on OpenCL library nor headers being available on the system at build time (we load the library dynamically at runtime, and we use our own copy of the headers), but we still check for both in configure and don't enable OpenCL support if either or both are missing.

Do we want to continue checking (I think @claudioandre-br said so)? If so, or regardless, do we possibly want explicit --enable-opencl to override any checks (just like --disable-opencl would)? I've just tested, and it does not yet do that, and indeed we currently only document --disable-opencl. We'll also need to document --enable-opencl if we start supporting it.

As to the default, if we do continue checking, then maybe check only for the library? We definitely don't need the headers anymore, at all, but the library we'd use at runtime, so it makes more sense to check for it. Alternatively, we could make the check an OR - enable OpenCL support by default if at least one of the two - library or headers - is found at build time, indicating that the system is possibly meant to work with OpenCL and is maybe just slightly misconfigured.

[solardiz]

if we do continue checking, then maybe check only for the library? We definitely don't need the headers anymore

We no longer check the headers due to @claudioandre-br's #5398.

We still check the library. We still do not have --enable-opencl that could be used to build with OpenCL on library-less systems - I think we should at least add that.

[solardiz]

@claudioandre-br I really would like to have --enable-opencl at least for build-testing of changes to OpenCL host code on systems without OpenCL.

[solardiz]

I really would like to have --enable-opencl at least for build-testing of changes to OpenCL host code on systems without OpenCL.

Also for --fuzz of OpenCL formats' hash parsing, including on CI, without needing to install an OpenCL library in there. Edit: this use case probably needs more changes, as we currently initialize an OpenCL device even during --fuzz of such formats.