gnu: openttd-gfx: Use new package style.

* gnu/packages/games.scm (openttd-gfx)[arguments]: Convert to list of
G-Expressions.
[native-inputs]: Drop labels.

gnu/packages/games.scm

@@ -4300,12 +4300,14 @@ engine.  When you start it you will be prompted to download a graphics set.")
          "0nhzlk6s73qvznm5fdwcs1b42g2plf26s5ag39fvck45zm7m48jk"))))
     (build-system gnu-build-system)
     (arguments
-     `(#:make-flags (list (string-append "CC=" ,(cc-for-target))
+     (list
+      #:make-flags
+      #~(list (string-append "CC=" #$(cc-for-target))
               (string-append "INSTALL_DIR="
-                                         (assoc-ref %outputs "out")
+                             #$output
                              "/share/games/openttd/baseset/opengfx"))
       #:phases
-       (modify-phases %standard-phases
+      #~(modify-phases %standard-phases
           (replace 'configure
             (lambda _
               ;; Make sure HOME is writable for GIMP.
@@ -4323,12 +4325,13 @@ engine.  When you start it you will be prompted to download a graphics set.")
        ;; to disable this test.
        #:tests? #f
        #:parallel-build? #f))
-    (native-inputs `(("dos2unix" ,dos2unix)
-                     ("gimp" ,gimp)
-                     ("grfcodec" ,grfcodec)
-                     ("nml" ,nml)
-                     ("which" ,which)
-                     ("python" ,python)))
+    (native-inputs
+     (list dos2unix
+           gimp
+           grfcodec
+           nml
+           which
+           python))
     (home-page "http://dev.openttdcoop.org/projects/opengfx")
     (synopsis "Base graphics set for OpenTTD")
     (description