net-libs/xulrunner-1.9.2.3-r1 can not be compiled with libpng-1.4.0 or greater. Im using ~amd64. Hotfix is to mask >=libpng-1.4.0. When you have emerged >=libpng-1.4.0, you can also not run firefox anymore, that's why it is a major bug in my opinion. That's the compile-error: "checking for gtk+-2.0 >= 2.10.0 gtk+-unix-print-2.0 glib-2.0 gobject-2.0 gdk-x11-2.0... Package libpng12 was not found in the pkg-config search path. Perhaps you should add the directory containing `libpng12.pc' to the PKG_CONFIG_PATH environment variable Package 'libpng12', required by 'cairo', not found configure: error: Library requirements (gtk+-2.0 >= 2.10.0 gtk+-unix-print-2.0 glib-2.0 gobject-2.0 gdk-x11-2.0) not met; consider adjusting the PKG_CONFIG_PATH environment variable if your libraries are in a nonstandard prefix so pkg-config can find them." Reproducible: Always
Use revdep-rebuild after libpng upgrade: in this case - reemerge cairo.