The build instructions for Illumos should be followed to build the base ONNV replacement required for OpenIndiana.
The OI version of illumos-gate is here:
Once the Illumos setup has been followed, nightly illumos.sh will initiate the build, if you want to follow the build as it occurs use
nightly illumos.sh & sleep 1; tail -f log/nightly.log
Note that OpenIndiana Hipster uses virgin illumos-gate, coming from https://github.com/illumos/illumos-gate .
Some modifications are made on top of it by illumos-gate oi-userland component . Using this component is the "official" way of building illumos-gate for Hipster.