components/alsa-lib.bst: Do not use host paths in Flatpak

We detect when we run within Flatpak and in this case we ignore variables ALSA_CONFIG_DIR and ALSA_CONFIG_PATH. Instead we use variables prefixed with FLATPAK_ so that application still have a way to provide a different configuration.

We disable loading ~/.asoundrc and instead load ${XDG_CONFIG_HOME}/asound.conf. ~/.asoundrc might not be valid for Flatpak and fail to load when taken from host when using --filesystem=home. A XDG path is safer. Users can then provide a asound.conf per application in .var/app/<app>/config/.

Part of #857 (closed)