It is somewhat difficult to remove something when installed this way, AFAIK. Next time, you might want to use checkinstall to generate a slackware package or use native slackware packages from a site like http://www.linuxpackages.net.
Or you can do something like this:
./configure||make||whatever||don't install anthing yet
cd / && find >system.find && cd -
make install (or whatever and include any other commands that will install files on the box)
cd / && find>system.find~ && diff system.find* >packagename.find && cd -
Anyways, you may need to update the slocate database before using the locate or slocate command.
Just run updatedb as root.