How can I add a package outside the guix source dir?

[Feng Shu <tumashu@163.com>]

I want to add a package which will used by myself, for example:

I don't want to put it to guix source dir, How can I solve the problem?

#+BEGIN_COMMENT
(define-public emacs-snapshot
  (package (inherit emacs)
           (name "emacs-snapshot")
           (version "20150522.9e41e0b")
           (source (origin
                     (method git-fetch)
                     (uri (git-reference
                           (url "/home/feng/project/emacs")
                           (commit "9e41e0bc6a6fdb7a01841db62d8de42d67be534a")))
                     (sha256
                      (base32
                       "1p50nrmwnx1d6an7daz8fxkj6ylxml7g23b996ba837nlfz3l8ay"))))
           (native-inputs
            `(("autoconf" ,autoconf)
              ("automake" ,automake)
              ,@(package-native-inputs emacs)))
           (arguments
            (substitute-keyword-arguments
                `(;; Build fails if we allow parallel build.
                  #:parallel-build? #f
                                    ;; Tests aren't passing for now.
                                    #:tests? #f
                                    ,@(package-arguments emacs))
              ((#:phases phases)
               `(modify-phases ,phases
                  (add-after 'unpack 'autogen
                             (lambda _
                               (zero? (system* "sh" "autogen.sh"))))))))))
#+END_COMMENT



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