You're right, my mistake.
I was rereading your post when I saw you pasted this: /usr/include/bits/local_lim.h:36:26: linux/limits.h: No such file or directory
And then it comes the sanity check error so I assume that missing file is what's causing it. A quick look at http://slackware.com/pb
showed that the file comes with glibc package and kernel-headers package (as kodon pointed out), but the first is /usr/include/limits.h while the kernel-headers one is /usr/include/linux/limits.h which is the one you seem to be missing and I guess it's needed by cpp too. Some programs requiere kernel-headers as you can see, but not usually.