Both the track player and the codec library decode FLAC files by invoking the official libFLAC API, and need libFLAC to be installed before they can be built with FLAC support. If you are not interested in playing FLAC files or stream audio in FLAC format, but have other uses for the Klipspringer hub, it’s possible to run without FLAC support. In that case, just ignore warnings about it from configure.
On many systems, libFLAC is simple to install using the package manager. For instance, on Debian-based Linuxes (such as Ubuntu and Raspberry Pi OS, you can just enter
sudo apt install libflac-dev
in the terminal. On macOS it’s almost as easy if you use Homebrew: use the command
brew install flac
(which you would probably have to do anyway to use FLAC any way at all).
If none of these installation methods works on your system, please refer to the official FLAC download page for more suggestions.