Fixes a build issue on pre-10.x FreeBSD systems, where MK_CTF is not def...

...ined (by /usr/share/mk/bsd.own.mk) yet.

The default on 9.1 and 10-CURRENT as of now is that CTF information is not used. With this patch, if MK_CTF is
undefined, the same logic as for MK_CTF == "no" will be employed.

PS.: Thanks at marino from IRC for hinting MK_CTF probably being undefined.