#9 (the fix for #8) broke the build when using the 10.9 SDK and older. You now unconditionally include a copy of vecLib.h that came from the macOS 11 SDK, which is substantially different from the version of that file required for earlier OS versions, causing this build failure:

vecLib-760.100.h:48:91: error: Accelerate/../Frameworks/vecLib.framework/Headers/LinearAlgebra/LinearAlgebra.h: No such file or directory

(There is no LinearAlgebra.h in 10.9 and earlier.)

Since that macOS 11 version of vecLib.h was added exclusively to fix build failure on the macOS 12 (or later?) SDK, I suggest you only include that file when using the macOS 12 (or later?) SDK. I'll submit a PR to fix it.