case $THIS_SUB_DEPENDS in
  ASF) [[ $TAGLIB_ASF == "-DWITH_ASF=ON" ]] && return 0;;
  MP4) [[ $TAGLIB_MP4 == "-DWITH_MP4=ON" ]] && return 0;;
  *) echo "Eek, unknown subdependency requested!"; return 1;;
esac
return 1