gnu: Use 'ghostscript-gs' in packages that need the 'gs' command.

Fixes a regression introduced in
eb354bdacb.

Reported by Andreas Enge <andreas@enge.fr>.

* gnu/packages/gv.scm (gv)[propagated-inputs]: Change from GHOSTSCRIPT/X
to GHOSTSCRIPT-GS/X.
* gnu/packages/music.scm (lilypond): Use GHOSTSCRIPT-GS instead of
GHOSTSCRIPT.
* gnu/packages/ocaml.scm (unison): Likewise.
* gnu/packages/plotutils.scm (asymptote): Likewise.
* gnu/packages/scheme.scm (chez-scheme): Likewise.

gnu/packages/gv.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
-;;; Copyright © 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -39,7 +39,7 @@
             (sha256 (base32
                      "0q8s43z14vxm41pfa8s5h9kyyzk1fkwjhkiwbf2x70alm6rv6qi1"))))
    (build-system gnu-build-system)
-   (propagated-inputs `(("ghostscript" ,ghostscript/x)))
+   (propagated-inputs `(("ghostscript" ,ghostscript-gs/x)))
    (inputs `(("libx11" ,libx11)
              ("libxaw3d" ,libxaw3d)
              ("libxinerama" ,libxinerama)

gnu/packages/music.scm

@@ -404,7 +404,7 @@ interface.  It is implemented as a frontend to @code{klick}.")
        ("font-tex-gyre" ,font-tex-gyre)
        ("fontconfig" ,fontconfig)
        ("freetype" ,freetype)
-       ("ghostscript" ,ghostscript)
+       ("ghostscript" ,ghostscript-gs)
        ("pango" ,pango)
        ("python" ,python-2)))
     (native-inputs

gnu/packages/ocaml.scm

@@ -569,7 +569,7 @@ libpanel, librsvg and quartz.")
     (native-inputs
      `(("ocaml" ,ocaml)
        ;; For documentation
-       ("ghostscript" ,ghostscript)
+       ("ghostscript" ,ghostscript-gs)
        ("texlive" ,texlive)
        ("hevea" ,hevea)
        ("lynx" ,lynx)))

gnu/packages/plotutils.scm

@@ -186,7 +186,7 @@ colors, styles, options and details.")
     ;; "help" command in interactive mode, so adding a "doc" output is not
     ;; currently useful.
     (native-inputs
-     `(("gs" ,ghostscript)              ;For tests
+     `(("gs" ,ghostscript-gs)           ;For tests
        ("texinfo" ,texinfo)             ;For generating documentation
        ("texlive" ,texlive)             ;For tests and documentation
        ("emacs" ,emacs-minimal)

gnu/packages/scheme.scm

@@ -635,7 +635,7 @@ threads.")
        ("stex" ,stex)))
     (native-inputs
      `(("texlive" ,texlive)
-       ("ghostscript" ,ghostscript)
+       ("ghostscript" ,ghostscript-gs)
        ("netpbm" ,netpbm)))
     (outputs '("out" "doc"))
     (arguments