Building libpng using the recipe libpng_1.6.17.bb will always issue the following warning: WARNING: Failed to fetch URL http://downloads.sourceforge.net/project/libpng/libpng16/1.6.17/libpng-1.6.17.tar.xz, attempting MIRRORS if available because the link that does not exist any more.
Looks like upstream moves the previous tarballs to "old-releases/" when it uploads new ones, so this is going to constantly hit us.
An alternative source could be: ftp://ftp.simplesystems.org/pub/png/src/libpng16 However, even that repository keeps moving the previous tarballs to ftp://ftp.simplesystems.org/pub/png/src/history/libpng16/
GENTOO_MIRROR seems to be the proper place to fetch the libpng source. Patch for master branch sent to oe-core ML: http://lists.openembedded.org/pipermail/openembedded-core/2015-December/114091.html
Fixed in master branch. Will sent patch for Jethro.
http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?h=jethro&id=655c8a5c9d1c2174e0c5221411937272281d09c7
Verified on master and jethro.