Fix for gcc6-aux on FreeBSD-12/AMD64

It builds fine and I'm inclined to commit it (without the MAINTAINER change for now unless ericturgeon confirms that he is ok with it). Any reason to wait?
The only reason is courtesy to Eric, but he privately pre-agreed to be the maintainer of gcc6-aux.
I think it would be fine to commit as-is and send Eric an email that it's already done.