test.log

$ sassc test.scss 
.fun-exists1 {
  exists: false;
  compact: true; }

.fun-exists2 {
  exists: false;
  compact: compact(true); }

test.scss

@if not function-exists(compact) {
  @function compact($vars...) {
    $list: ();
    @each $var in $vars {
        @if $var {
            $list: append($list, $var, comma);
        }
    }
    @return $list;
  }

  .fun-exists1 {
    @if function-exists(compact) {
      exists: true;
    }
    @else {
      exists: false;
    }
    compact: compact(true);
  }
}

.fun-exists2 {
  @if function-exists(compact) {
    exists: true;
  }
  @else {
    exists: false;
  }
  compact: compact(true);
}