[css] Disable LESS-CSS Overwriting calc()

Here's a cross-browser less mixin for using CSS's calc with any property:

.calc(@prop; @val) {
  @{prop}: calc(~'@{val}');
  @{prop}: -moz-calc(~'@{val}');
  @{prop}: -webkit-calc(~'@{val}');
  @{prop}: -o-calc(~'@{val}');
}

Example usage:

.calc(width; "100% - 200px");

And the CSS that's output:

width: calc(100% - 200px);
width: -moz-calc(100% - 200px);
width: -webkit-calc(100% - 200px);
width: -o-calc(100% - 200px);

A codepen of this example: http://codepen.io/patrickberkeley/pen/zobdp