FreeBSD's knobs are similar to Gentoo's global USE flags that can be set in make.conf.
See /usr/ports/KNOBS for global variables that can be set.
You can add these to /etc/make.conf to enable / disable them.
The variables WITH_FOO or WITHOUT_BAR only need to be defined to any setting to be effective. Common practice is to simply use yes.
WITHOUT_X11=yes