I read the thread and can happily report that I tried building Mate on a clean slackwareARM load on my RaspberryPi3. I used the slackbuild for it that's from
https://github.com/mateslackbuilds/msb. The answer as to why things happened the way they did is beyond me if I read the thread correctly so I'll just leave it as I've said twice; the slackbuild wanted libstdc++.la, I gave it one, it build. I wish I could remember the package that didn't build correctly but I do know it was near he end of the list is 'base'.
Jim