Patch is trivial.
Created attachment 222909 [details, diff] patch that fixes the problem
thanks + 09 Mar 2010; Samuli Suominen <ssuominen@gentoo.org> prboom-2.5.0.ebuild, + +files/prboom-2.5.0-libpng14.patch: + Fix building with libpng14 wrt #308739 by Rafał Mużyło.