Did you do a fresh install or upgrade from 4.x?
I did the latter and it took a bit of doing... assuming you've updated /boot, run mergemaster at the appropiate places, moved /usr/include if left over from 4.x, and have a running GENERIC kernel.
If installworld fails anywhere, do a make all install in the offending directory, go back up to /usr/src and run installworld.
Once that is done clean everything out, do the whole buildworld, buildkernel, installkernel, installworld dance again.
If you get it all in one run with no errors, then work on tweaking your kernel.
Kind of convoluted but it worked for me, hope it helps.
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