Generate C Driver version number when installing

In Evergreen we clone libmongoc master from git and install it before building the C++ driver. Starting with CDRIVER-2831 we need to call two libmongoc scripts before building libmongoc with cmake:

python build/calc_release_version.py > VERSION_CURRENT

python build/calc_release_version.py -p > VERSION_RELEASED

Otherwise, cmake sets libmongoc's version number to 0.0.0, which doesn't meet the C++ Driver's minimum required libmongoc version.