gnu: graphviz: Add "doc" output.

* gnu/packages/graphviz.scm (graphviz)[outputs, arguments]: New fields.

gnu/packages/graphviz.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -59,7 +60,16 @@
                    (substitute* "lib/gvc/Makefile"
                      (("am__append_5 *=.*")
                       "am_append_5 =\n")))
-                 %standard-phases)))
+                 (alist-cons-after
+                  'install 'move-docs
+                  (lambda* (#:key outputs #:allow-other-keys)
+                           (let ((out (assoc-ref outputs "out"))
+                                 (doc (assoc-ref outputs "doc")))
+                             (mkdir-p (string-append doc "/share/graphviz"))
+                             (rename-file (string-append out "/share/graphviz/doc")
+                                          (string-append doc "/share/graphviz/doc"))
+                             #t))
+                 %standard-phases))))
     (inputs
      `(("libXrender" ,libxrender)
        ("libX11" ,libx11)
@@ -76,6 +86,7 @@
     (native-inputs
      `(("bison" ,bison)
        ("pkg-config" ,pkg-config)))
+    (outputs '("out" "doc"))                      ; 5 MiB of html + pdfs
     (home-page "http://www.graphviz.org/")
     (synopsis "Graph visualization software")
     (description