Added clang inline helper
gcc on macOS I think executes clang more often than not, and it appears we didn't have a inline helper defined for that. This is ported from `CYTHON_INLINE` which essentially does the same in cythonized files, save the static declaration

This might add a bit of build time to macOS (I think partially explains why it's a few minutes faster than the rest) but makes it more similar in performance to the others

Running the full benchmark suite to see where this may help