While attempting to upgrade to the latest version of Trac from the FreeBSD ports you may encounter the above error. Finding a solution for this problem isn't as easy as it may seem and so I decided I'd put it here to help out a few lost souls.
If you receive this error then it's most likely that you've built subversion with the STATIC make option. Building a static version means that no shared libraries will be built and this is the reason you get the svn_client-1 does not exist error. The solution is to:
cd /usr/ports/devel/subversion && make config
Ensure you untick the STATIC option then rebuild and reinstall the subversion port. Once you've done this you can return to your trac build and live in happiness once again.