patch was straight forward (seems like the code is largely the same as hugin!?), but someone who knows png/c++ should double check
Created attachment 263347 [details, diff] enblend-4.0-libpng-1.5.patch
+ 07 Aug 2011; Samuli Suominen <ssuominen@gentoo.org> enblend-4.0.ebuild, + +files/enblend-4.0-libpng15.patch: + Fix building with libpng15 wrt #355893 by Mike Frysinger