Using Compass to generate normal and retina sprite maps at once

_compass-retina-sprites.scss

@mixin all-retina-sprites($map, $map2x) {
  @media (min--moz-device-pixel-ratio: 1.5),
    (-o-min-device-pixel-ratio: 3/2),
    (-webkit-min-device-pixel-ratio: 1.5),
    (min-device-pixel-ratio: 1.5),
    (min-resolution: 1.5dppx) {

    $base-class: sprite-map-name($map);

    .#{$base-class}-all-retina-sprites {
      background-image: sprite-url($map2x);
    }

    @each $sprite in sprite-names($map) {
      .#{$base-class}-#{$sprite} {
        @extend .#{$base-class}-all-retina-sprites;
        @include sprite($map2x, $sprite);
        @if (image-width(sprite-file($map, $sprite)) != image-width(sprite-file($map2x, $sprite))) {
          @include background-size(image-width(sprite-file($map, $sprite)) auto);
        }
      }
    }

  }
}

_sprites.scss

// Sprite set: General
// ----------------------------------------
//$general-layout: smart;
$general-spacing: 1px;
$general-sprite-dimensions: true;
@import "general/*.png";
@include all-general-sprites;

$general-retina-spacing: 1px;
$general-retina-sprite-dimensions: true;
@import "general-retina/*.png";
//@include all-general-retina-sprites;

@include all-retina-sprites($general-sprites, $general-retina-sprites);