Add compact if not defined

lib/compass/functions/_lists.scss

@@ -76,3 +76,15 @@
 @function first-value-of($list) {
   @return nth($list, 1);
 }
+
+@if not(function-exists(compact)) {
+  @function compact($vars...) {
+    $list: ();
+    @each $var in $vars {
+        @if $var {
+            $list: append($list, $var, comma);
+        }
+    }
+    @return $list;
+  }
+}