gnu: libpng: Upgrade to 1.5.17.

* gnu/packages/libpng.scm (libpng): Upgrade to 1.5.17.

gnu/packages/libpng.scm

@@ -27,21 +27,24 @@
 (define-public libpng
   (package
    (name "libpng")
-   (version "1.5.14")
+   (version "1.5.17")
    (source (origin
             (method url-fetch)
-            (uri (string-append
-                   "http://downloads.sourceforge.net/project/libpng/libpng15/"
-                   version "/libpng-"
-                   version ".tar.xz"))
-            (sha256 (base32
-                     "0m3vz3gig7s63zanq5b1dgb5ph12qm0cylw4g4fbxlsq3f74hn8l"))))
+
+            ;; Note: upstream removes older tarballs.
+            (uri (list (string-append "mirror://sourceforge/libpng/libpng15/"
+                                      version "/libpng-" version ".tar.xz")
+                       (string-append
+                        "ftp://ftp.simplesystems.org/pub/libpng/png/src"
+                        "/libpng15/libpng-" version ".tar.xz")))
+            (sha256
+             (base32 "19wj293r4plbfgb43yhrc2qx8bsch9gbazazfqrj9haa7lsk29jp"))))
    (build-system gnu-build-system)
 
    ;; libpng.la says "-lz", so propagate it.
    (propagated-inputs `(("zlib" ,zlib)))
 
-   (synopsis "Libpng, a library for handling PNG files")
+   (synopsis "Library for handling PNG files")
    (description
     "Libpng is the official PNG (Portable Network Graphics) reference
 library. It supports almost all PNG features and is extensible.")