Upstream bug has patches linked: http://trac.osgeo.org/gdal/ticket/3914 Should also be fixed with upcoming 1.8.1 version.
Created attachment 264259 [details] gdal-1.8.0-r1 ebuild This new ebuild patches GDAL-1.8.0 to support libpng 1.5. It also add supports for the GeoPDF format, which is triggered by the "poppler" USE flag (see http://bugs.gentoo.org/show_bug.cgi?id=354077#c2 ).
Created attachment 264261 [details, diff] Patch to add libpng-1.5 support in GDAL-1.8.0 Provenance : upstream patch (http://trac.osgeo.org/gdal/changeset/21526)
Updated, test away...