This contains references to various external repositories which are known to build and work properly with the current libavif. If you are building for Windows or any kind of fully static (embedded) release, using these scripts in conjunction with the BUILD_SHARED_LIBS=0 and AVIF_* CMake flags make for a convenient way to get all of the dependencies necessary. This method is how many of libavif's continuous builders work.
However, if you are building this for a distribution or Unix-like environment in general, you can safely ignore this directory. Allow BUILD_SHARED_LIBS to keep its default (ON), enable the appropriate AVIF_BUILD_* CMake flags depending on which shared AV1 codec libraries you plan to leverage and depend on, and use the system's zlib, libpng, and libjpeg.