Summary:
Make `build-oss.sh` actually run the cmake build.
At the moment this is only run from nbtd for mac platforms,
so that is all that I've tested with it.
This is similar to the the equivalent scripts for watchman.
In a later diff we can centralize and clean up the duplication.
Reviewed By: simpkins
Differential Revision: D13910031
fbshipit-source-id: 6a7250c22033e913d999cf928fed27dada0647ef