first attempt failed (nested /* */ in header comment terminated the block early):
  regvifnull.c:13:75: error: expected identifier or '(' before ')' token
after fix:
$ cd /root/mrt && cc -O2 -Wall -o regvifnull regvifnull.c
(exit 0, only the unused-variable warning from the common header users)
$ ls -la regvifnull
-rwxr-xr-x 1 root  wheel  ... /root/mrt/regvifnull