In /usr/bin/freealut-config line 53:
  -*=*) optarg=`echo "$1" | sed 's/[-_a-zA-Z0-9]*=//'` ;;
               ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
  -*=*) optarg=$(echo "$1" | sed 's/[-_a-zA-Z0-9]*=//') ;;

For more information:
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...