slackware & kernel header files
you cannot compile kernel module.
In fact it is highly possible you to don't be able to compile anything. The problem is that gcc cannot find the linux include headers.
By some reason Slack8.1 & 90 install kernel headers directly into /usr/include, and not as usuall in /usr/src/linux/include. There are 2 ways to fix that. I do not recomend you to install the kernel-headers package if you don't use the distro kernel.
The right way to fix it is to do:
ln -s /usr/src/linux/include/linux /usr/include/linux
ln -s /usr/src/linux/include/asm /usr/include/asm
usually /usr/src/linux is symlink to the current kernel (e.g. /usr/src/linux-2.4.20)
I've mailed slackware support with details of this (common) problem, so it should be fixed next release).