I just did a routine update on my ~amd64 machine and saw the portage warning that libpng14 has been replaced by libpng15, and I should run revdep-rebuild --library '/usr/lib/libpng14.so' and then delete the obsolete library.
That's what I did. I confess I wasn't watching, so I may have missed some important errors during the run. After that I ran plain revdep-rebuild as I do after every update, and saw that two gnome packages failed to rebuild properly because lpng14 couldn't be found :/ >From painful experience I've learned that good-old libtool files (*.la) are the usual suspects, and grep found -lpng14 in about ten .la files even after both revdep-rebuilds. Grrr! This fixed the problem for me (as similar moves have done in the past): #find /usr/lib64 -name \*.la -exec sed -i s/png14/png15/ '{}' ';' I ran revdep-rebuild again and the two broken packages emerged nicely this time. I hope no one else will hit this problem, but at least this is a good workaround if you do.