How to replace %output with #$output?

[Mekeor Melire]

In the IRC-channel, I was told that the undocumented, magic %output
variable is deprecated. Instead, I should use #$output. But I couldn't
get this done. I always ran into strange errors.

How should I change the following snippet?

--8<---------------cut here---------------start------------->8---
    (arguments
      '(
         #:tests? #f ; no tests
         #:make-flags (list "CC=gcc")
         #:phases
         (modify-phases %standard-phases
             (delete 'configure)
             (add-after 'unpack 'patch-makefile
               (lambda _
                 (substitute* "Makefile"
                   (("/usr/local") %output)))))))
--8<---------------cut here---------------end--------------->8---

The whole file is available here: http://ix.io/3T71/scm