Upstream bug has patches linked:
Should also be fixed with upcoming 1.8.1 version.
Created attachment 264259 [details]
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...