The problem is with that particular snapshot of MPlayer that you are trying to build.
Apparently, the part that gets stripped when building the Slackware package without patent issues, contains a bit of code which was added recently and which prevents successful compilation.
This has more information about your "error: static declaration of 'log2' follows non-static declaration" issue (look it up in your build log): http://lists.mplayerhq.hu/pipermail/...ly/077259.html
Using the latest SVN snapshot of MPlayer fixes your problem
Look for the VERSION definition in MPlayer.SlackBuild and change the value "r29390
" to either one of these: "r29709
" or "20090923
". Both will checkout today's snapshot. Using this version, the MPlayer.SlackBuild script will compile a package with no errors.