Add kernel build fixes.

I got rid of the KERNEL_HEADERS_INSTALL thing. Kind of silly. The
user is just expected to set their prefix right. We still have to
do a hook in order to install the kernel-specific stuff. libcap.a
and libcap.ko (kernel binaries) go in prefix/lib.
1 job for master
Status Job ID Name Coverage
  Test
passed #164
testing

00:00:18