I have been waiting for the next 2.0 release as i'm also the current maintainer for Wesnoth for SlackBuilds. If it's possible, i want to make sure Wesnoth 2.0 is buildable on Slackware 13.1 and also in the next Slackware 13.37 release. Meanwhile, please grab the 1.8.5 version on SlackBuilds. You can also try to use the same SlackBuild to compile 1.9.4, the latest development version, but you need to comment one line that is the patch required for 1.8.4 for libpng. It's no longer used for 1.9.4 as the patch has been included in the codebase.
patch -p1 < $CWD/wesnoth-libpng-1.4.0.patch