bug#16771: 24.3.50; pcase generates irritating warnings

[Helmut Eller <eller.helmut@gmail.com>]

Compiling this example:
  ;; -*-lexical-binding:t-*-
  (eval-when-compile
    (require 'pcase))
  
  (defun foo (x)
    (pcase x
      ((and `(,sym)
            (guard (symbolp sym)))
       t)))

with emacs -batch -f batch-byte-compile foo.el produces this output:

  In toplevel form:
  foo.el:5:1:Warning: Unused lexical variable `sym'
  Wrote /tmp/foo.elc

This warning is annoying as sym is obviously used in the guard
expression and replacing sym with _sym also doesn't help because that
produces

  foo.el:5:1:Warning: variable `_sym' not left unused

In GNU Emacs 24.3.50.6 (i686-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2014-02-14 on ix