rebirth: relibprebirth: Negate unary -

* build-aux/bootstrap/rebirth/relibprebirth.scm (-): Negate when given only
    a single argment.

build-aux/bootstrap/rebirth/relibprebirth.scm

@@ -142,10 +142,12 @@
            0
            xs))
    (define (- . xs)
+     (if (= (length xs) 1)
+         (- 0 (car xs))
          (fold (lambda (y x)
                  (string->es "$$x - $$y"))
                (car xs)
-           (cdr xs)))
+               (cdr xs))))
    (define (zero? x)
      (eq? x 0))