Add
MAKEOPTS='-j8'
MAKEFLAGS='-j8'
to /etc/make.conf
(create it if it doesn't already exist).
If that doesn't work, add
export MAKEOPTS='-j8'
export MAKEFLAGS='-j8'
to your system-wide profile (e.g., /etc/profile).
For me, MAKEOPTS alone didn't work. Possibly MAKEFLAGS is all that's needed.