gnu: tzdata-for-tests: Update to 2018c.

* gnu/packages/base.scm (tzdata-for-tests): Replace with a simple hidden copy
of 'tzdata'.

gnu/packages/base.scm

@@ -1161,63 +1161,7 @@ and daylight-saving rules.")
 ;;; will typically be obsolete and should never be referred to by a built
 ;;; package.
 (define-public tzdata-for-tests
-  (hidden-package (package (inherit tzdata)
-    (version "2017c")
-    (source
-      (origin
-        (method url-fetch)
-        (uri (string-append "https://www.iana.org/time-zones/repository"
-                            "/releases/tzdata" version ".tar.gz"))
-        (sha256
-         (base32
-          "02yrrfj0p7ar885ja41ylijzbr8wc6kz6kzlw8c670i9m693ym6n"))))
-    (arguments
-     '(#:tests? #f
-       #:make-flags (let ((out (assoc-ref %outputs "out"))
-                          (tmp (getenv "TMPDIR")))
-                      (list (string-append "TOPDIR=" out)
-                            (string-append "TZDIR=" out "/share/zoneinfo")
-
-                            ;; Discard zic, dump, and tzselect, already
-                            ;; provided by glibc.
-                            (string-append "ETCDIR=" tmp "/etc")
-
-                            ;; Likewise for the C library routines.
-                            (string-append "LIBDIR=" tmp "/lib")
-                            (string-append "MANDIR=" tmp "/man")
-
-                            "AWK=awk"
-                            "CC=gcc"))
-       #:modules ((guix build utils)
-                  (guix build gnu-build-system)
-                  (srfi srfi-1))
-       #:phases
-       (modify-phases %standard-phases
-         (replace 'unpack
-           (lambda* (#:key source inputs #:allow-other-keys)
-             (and (zero? (system* "tar" "xvf" source))
-                  (zero? (system* "tar" "xvf" (assoc-ref inputs "tzcode"))))))
-         (add-after 'install 'post-install
-           (lambda* (#:key outputs #:allow-other-keys)
-             ;; Move data in the right place.
-             (let ((out (assoc-ref outputs "out")))
-               (symlink (string-append out "/share/zoneinfo")
-                        (string-append out "/share/zoneinfo/posix"))
-               (delete-file-recursively
-                (string-append out "/share/zoneinfo-posix"))
-               (copy-recursively (string-append out "/share/zoneinfo-leaps")
-                                 (string-append out "/share/zoneinfo/right"))
-               (delete-file-recursively
-                (string-append out "/share/zoneinfo-leaps")))))
-         (delete 'configure))))
-    (inputs `(("tzcode" ,(origin
-                          (method url-fetch)
-                          (uri (string-append
-                                "http://www.iana.org/time-zones/repository/releases/tzcode"
-                                version ".tar.gz"))
-                          (sha256
-                           (base32
-                            "1dvrq0b2hz7cjqdyd7x21wpy4qcng3rvysr61ij0c2g64fyb9s41")))))))))
+  (hidden-package tzdata))
 
 (define-public libiconv
   (package