/**
 * Escape and wrap a message value as literal string for LESS.
 *
 * This mostly lets CSSMin escape it and wrap it, but also escape single quotes
 * for compatibility with LESS's feature of variable interpolation into other strings.
 * This is relatively rare for most use of LESS, but for messages it is quite common.
 *
 * Example:
 *
 * @code
 *     @x: "foo's";
 *     .eg { content: 'Value is @{x}'; }
 * @endcode
 *
 * Produces output: `.eg { content: 'Value is foo's'; }`.
 * (Tested in less.php 1.8.1, and Less.js 2.7)
 *
 * @param string $msg
 * @return string wrapped LESS variable value
 */
private static function wrapAndEscapeMessage( $msg ) {
	return str_replace( "'", "\'", CSSMin::serializeStringValue( $msg ) );
}