That is a known problem with SDL 1.2.14. There is also a corresponding entry in Wesnoth bugzilla:
It points to this bug report on SDL bugzilla:
You'll find attached a patch accepted by upstream. I have tested briefly with Wesnoth, Dosbox and DosEmu and it worked with out any problems.
If you can't get around, I uploaded the modified SlackBuild with all needed files here:
If you decide not to trust me, you can fetch only sdl.SlackBuild and SDL_x11events.c.diff.gz, compare (diff) them with the original SlackBuild
and then get the rest of files from an official Slackware mirror like:
Whatever fits you the best. Anyway, I hope this helps.
Btw, I reported the bug, so it might get resolved soon.