I had to add the line
#define PCIR_HDRTYPE PCIR_HEADERTYPE
to the block between #if __FreeBSD_version >= 500000 and its #endif in nv-freebsd.h because otherwise the build would hang up at nvidia_sysctl.c.
I don't know if this trick was included in somewhere in documentation or if the driver even works, because it won't use the DVI output in my card. (see my post about this at http://www.nvnews.net/vbulletin/showthread.php?t=34442
) My own problem is a matter of configuration but I still need help with it.