macOS: prebuilt binaries don't work

[agarny]

I wanted to use the prebuilt version of libCellML for macOS. I can install it fine (after telling macOS to ignore its security concerns!). From there, I was able to get a small CMake file to find my installed copy of libCellML, but... then CMake complains about /Applications/Xcode_15.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.2.sdk/usr/include being non-existent on my system. Well, indeed, I don't have such a folder on my system!

The problem is that libCellML's CMake file have that path hard-coded. So, unless we have exactly the same settings as the machine that was used to generate libCellML's package, we can't just download libCellML and use it on macOS...