I'm not sure it's a good idea to mix headers from a different kernel version with the libc headers in /usr/include. The former are for building kernel modules, and the latter for userland programs that link against glibc. If some of the system calls changed between the version of your custom kernel and the version glibc was built against, then you can get problems when those calls are made.
I had a lot of problems when I added a 32-bit building environment to my 64-bit Slackware system. I ended up just installing a 32-bit system under VirtualBox and doing my PBSDK builds there. It's a bit more hassle, but fewer headaches.