“Failed to configure php55-mysql” with MacPorts and Percona 5.5
This article How to Update a Percona Installation gave me the clue I needed to fix the problem. I'm not going to delve into it here but the gist is that Percona had to rename libmysqlclient to libperconaserverclient. The fix in that article doesn’t apply to OS X/MacPorts but this fixed the issue.

cd /opt/local/lib/percona/mysql
sudo ln -s libperconaserverclient.a libmysqlclient.a
sudo ln -s libperconaserverclient.dylib libmysqlclient.dylib


Then complete your install/update.