Here are the steps I took to conplete the update of libpng on Gentoo

emerge --sync


Before updating from libpng-1.2 to libpng-1.4 make sure that you have
fetched any other files that need to be updated by running "emerge -euNDf world".

You may need these files in advance because updating libpng is going to
temporarily break many applications linked to it, especially if you're
a Gnome desktop user.


emerge -euNDf world


Lets get started.

emerge --oneshot libpng


* Moving from libpng 1.2.x to 1.4.x will break installed libtool .la
* files.

* Run /usr/sbin/libpng-1.4.x-update.sh at your own risk only if
* revdep-rebuild or lafilefixer fails.

* Don't forget "man emerge" and useful parameters like --skip-first,
* --resume and --keep-going.


cat /usr/sbin/libpng-1.4.x-update.sh
#!/bin/bash

echo "Try revdep-rebuild or lafilefixer first."
echo "This script will rename -lpng12 and libpng12.la to -lpng14 and libpng14.la"
echo "in your system libdir libtool .la files without asking permission."

[[ -d /usr/lib64 ]] && lib_suffix=64

libdir=/usr/lib${lib_suffix}

find ${libdir} -name '*.la' | xargs sed -i -e '/^dependency_libs/s:-lpng12:-lpng14:'
find ${libdir} -name '*.la' | xargs sed -i -e '/^dependency_libs/s:libpng12.la:libpng14.la:'

# WTFPL-2

As I am brave and this was a testing box I ran next;

find /usr/lib64 -name '*.la' | xargs sed -i -e '/^de /s:-lpng12:-lpng14:'


then;

lafilefixer --justfixit

then;

emerge -1 cairo pango gtk+ atk libglade qt-gui

Then for good measure :)

revdep-rebuild

revdep-rebuild rebuild 51 packages

Now you're ready for the "big rebuild". I had to rebuild 70 apps. Those
of you running Gnome, if you're no longer able to use or get to a terminal
in X then just 'Ctrl-Alt-F1' to drop to a shell.

Beware, that you may have to rebuild over 100 apps including Open Office if it
was built with Gnome support.

After this is all done 'Ctrl-Alt-F7' will get you back into your desktop
environment.


emerge -uND world


Then run 'revdep-rebuild' once again. When finished your system should be sane again.

revdep-rebuild

So to make a long story short this may work :)

emerge -uDN world && revdep-rebuild -- --keep-going && lafilefixer --justfixit && revdep-rebuild -- --keep-going