Patch is trivial - present in the upstream bug. For the moment, configure.ac will need adding libpng15 in the obvious place. Quite fortunately, once released, gtk-pixbuf 2.24.0 will move libpng to Requires.private, so the headache will be even smaller.
*** Bug 355871 has been marked as a duplicate of this bug. ***
vapier's build.log with libpng 1.5.x is here: https://bugs.gentoo.org/show_bug.cgi?id=355871
ugh, typo in summary makes it hard for searches to hit
This is the commit: http://git.gnome.org/browse/gdk-pixbuf/commit/?id=e6a5b2472a4a5d554b587dfcb798b95035caa6fd
Fixed in the tree, thanks for reporting.