It maybe EOL, but as you're still delivering your packages for this release, it might be worth fixing?
You have a mysql-server-5.5 in your repos, which needs perl-DBD-MySQL, which needs libmysqlclient.so.16. So, when there is no .so.16, there is no mysql-server-5.5, at least not for x86_64. i686 works fine.
This is a clean machine, nothing but distro- and atomic-RPMs.
[cceller@inch ~]$ cat /etc/fedora-release
Fedora release 13 (Goddard)
[cceller@inch ~]$ uname -i
[cceller@inch ~]$ yum repolist
atomic Fedora Core 13 - atomicrocketturtle.com
fedora Fedora 13 - x86_64
updates Fedora 13 - x86_64 - Updates
[cceller@inch ~]$ rpmbuild -bb rpmbuild/SPECS/mysqlclient16-art.spec
[cceller@inch ~]$ echo $?
Builds clean for me, from your own src.rpm
BUT: as long, as I don't set up my own little repo for packages like this, it is not possible to perform updates, due to the missing lib blowing up yum.
(And yes, we're currently migrating from FC13 to CentOS 6, so this will be fixed for me at least