That's not Qt, that's GTK3. Slackware 14.0 ships with both GTK2 and GTK3. Now if Transmission detects both, it opts for GTK3 as default. Up to Transmission 2.60 - which is the version I use - you can add the following option to ./configure:
I don't know if you use SBo's SlackBuild. If that is the case, you can simply comment out the part responsible for building the Qt interface, add the option above and only build the GTK interface.
Starting with Transmission 2.61, you won't be able to build against GTK2 anymore.