Um, /usr/include/sys/systm.h at line 169 says:

   #include <lib/libkern/libkern.h>

   This file doesn't exist anywhere but in the source tree... Are my default
   include paths set wrong (i.e. should they include /usr/sys/src) or what?

Um, what the Hell are you trying to do?  systm.h is only supposed to
be used inside the kernel.