Fix MSVC+OpenCL build
ClFFT has been broken on MSVC for years. Now, we have an alternative.
- More path-to-string conversion issues.
- Minor CMake hiccup with VkFFT-related option.
- Updated error message when compiling with ClFFT.
Verified with MSVC 2022 + Intel Gen 9.5 iGPU, tests pass.
Refs #4635 (closed), #4622