This package was built for Slackware 12.2 not 13.0. 12.2 has KDE3 libraries in /usr/* Slackware 13.0 has KDE3.x libraries in /opt/kde3/*.
Slackbuilds.org has a SlackBuild for the older version of Gwenview in their 12.2 repo
link. Edit the SlackBuild to reflect
your system.
hint
Export the correct QTDIR and change the --prefix=/usr to --prefix=/opt/kde3. Maybe more