libpng: use SourceForge mirror

The Gentoo mirror also deletes old versions when they're not used, so revert
back to the canonical SourceForge site, adding /older-releases/ to MIRRORS to
handle new releases moving the version we want.

Original idea by Maxin B. John <maxin.john@intel.com>.

(From OE-Core rev: 791a3493c88c9c249f21f6d893b2061e1d8a0af6)

(From OE-Core rev: 8ced3de463f97930404fe83a9f30d5d6536ffc9b)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
[Updated for Pyro context]
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-multimedia/libpng/libpng_1.6.28.bb

@@ -6,11 +6,15 @@ LIC_FILES_CHKSUM = "file://LICENSE;md5=67d8837410863f9821bbd606536f0329 \
                     file://png.h;endline=144;md5=abfa0497feb393b5842d3d82c1009520"
 DEPENDS = "zlib"
 
-SRC_URI = "${GENTOO_MIRROR}/libpng-${PV}.tar.xz \
-          "
+LIBV = "16"
+
+SRC_URI = "${SOURCEFORGE_MIRROR}/project/${BPN}/${BPN}${LIBV}/${PV}/${BP}.tar.xz"
+
 SRC_URI[md5sum] = "425354f86c392318d31aedca71019372"
 SRC_URI[sha256sum] = "d8d3ec9de6b5db740fefac702c37ffcf96ae46cb17c18c1544635a3852f78f7a"
 
+MIRRORS += "${SOURCEFORGE_MIRROR}/project/${BPN}/${BPN}${LIBV}/older-releases/${PV}"
+
 BINCONFIG = "${bindir}/libpng-config ${bindir}/libpng16-config"
 
 inherit autotools binconfig-disabled pkgconfig