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configure: error: C preprocessor "/lib/cpp" fails sanity check

I upgraded my Slackware 13 to Slackware current. I also compiled a new kernel. I've been using it for a week or so and everything seems fine.
The problem is that when I try to compile something, it gives me errors.

When I tried to compile qtparted, I got:

Code:

configure: error: C preprocessor "/lib/cpp" fails sanity check

Thank you

I also get the error:

Code:

In file included from /usr/include/bits/posix1_lim.h:153,

                from /usr/include/dirent.h:232,

                from resources.cpp:3:

/usr/include/bits/local_lim.h:39:26: error: linux/limits.h: No such file or directory

So I assume the headers or something are missing. I checked the slackware website and the testing section has got packages for kerel 30.5.

If I installed the kernel from source, do I still need to install any headers/sources? Isn't it just a question of making some symlinks?

I made a symlink (/usr/src/linux) of the kernel directory (/usr/src/linux-2.6.31.5). Was it a good thing to do?

Ok, it seems to be sorted. What I needed to do is install the headers for the old kernel. I don't actually understand it, but it works. If I'm running my custom kernel (31.5), why the hell do I need to install the headers for the kernel that I removed?
It really doesn't make sense to me.