to /etc/make.conf -- see /usr/share/examples/etc/make.conf for more
details. This will give you a SASL-enabled base system sendmail once
you rebuild it.
Is that rebuild as in cd /usr/src && make buildworld or
cd /usr/src/usr.sbin/sendmail && make
Note that the second choice makes your base system dependent on stuff
external to it, which may or may not be desirable.
Oh, and before anyone else chimes in with a knee-jerk reaction about
using sendmail: there are other MTAs that can provide this
functionality. As an alternative, either postfix or exim could do what
you want too. Sendmail works just fine for me though.