Currently, if no git refs are found, calc_release_version.py reports the version as 0.0.0 and building. This can happen for example, if doing a shallow clone of the repository.

$ git clone --depth=1 git@github.com:mongodb/mongo-c-driver.git 
$ cd mongo-c-driver
$ python ./build/calc_release_version.py
0.0.0

cmake should consider this an error.