portbuild.tcl: Specify -j flag when using ninja
portbuild.tcl: Specify -j flag when using ninja

ninja build in parallel by default, but this makes it honor the
macports.conf setting.

danchr/macports-base,neverpanic/macports-base,danchr/macports-base,neverpanic/macports-base,macports/macports-base,macports/macports-base