xemacs fails to build with new libpng

[Jeremy Lavergne]

> Now MacPorts won't let me deactivate libpng 1.5 because a number of ports are depending on it. Is there a way to roll back all of the ports to the versions that use libpng 1.4 while things are being sorted out? 
> 
> The "xemacs" program is something I use all the time.

The likely quick fix for this situation is to rebuild xemacs from source:
sudo port clean xemacs
sudo port -s upgrade --force xemacs

If something more intensive than linking against the new libpng libraries needs done, then you should see about setting up a local repository checked out from before libpng's upgrade:
https://trac.macports.org/wiki/howto/InstallingOlderPort