If you did a full install of slackware 11 then you most likely installed the 220.127.116.11 kernel to start with before you added the 18.104.22.168, then you have the 22.214.171.124 kernel headers files installed by default.
These files are installed to /usr/include/asm-generic /usr/include/asm
, if you don't remove or rename and point them to the 2.6 kernel header files, it will keep compiling your software using the 2.4 headers instead of the 2.6.
I will assume you have the 2.6 kernel source in the /usr/src directory, rename these directories so you can always change back if this is not what is causing the problem.
mv /usr/include/asm /usr/include/asm-2.4
mv /usr/include/asm-generic /usr/include/asm-generic-2.4
mv /usr/include/linux /usr/include/linux-2.4
Now change into the /usr/src directory and remove the symlink to the old 2.4 kernel.
rm -rf linux
ln -s 126.96.36.199 linux
Now symlink the 188.8.131.52 kernels headers to the /usr/include directory
ln -s /usr/src/linux/include/asm-generic /usr/src/linux/include/asm
ln -s /usr/src/linux/include/linux /usr/include/linux
ln -s /usr/src/linux/include/asm /usr/include/asm
ln -s /usr/src/linux/include/asm-generic /usr/include/asm-generic
Now compile your programs and see if that fixes the compiling errors. Not to say this is the cause, but I had similar issues the other week and found I still had 2.4 headers files installed from the original install, and some software compiled while others would not. Once I removes the files and created symlinks to the 2.6 headers files, I have had no problems compiling since.