Build with CMake, using the following options:

```
cmake .. -DCMAKE_OSX_ARCHITECTURES="x86_64;arm64" -DCMAKE_OSX_DEPLOYMENT_TARGET="10.12" -DBUILD_SHARED_LIBS=ON
```

And some minor tweaks with `install_name_tool -id` to make sure that the
resulting libFLAC.8.dylib referred to itself with @rpath and not full
paths of the build directory.

Version v1.3.1-101-g66d060c7 was used from the following repository:

https://github.com/xiph/flac.git

A single patch was used, and is included. Affects x86_64, but not arm64.

Also, x86_64 had to be built separately to enable the intrinsic
optimizations. Oops.