This doesn't change the behavior when `--jobs`/`-j` is specified as a
command-line argument or in `SCONSFLAGS`.
The SCons hack used to know if `num_jobs` was set by the user is derived
from the MongoDB setup.
We use `os.cpu_count()` for portability (available since Python 3.4).
With 4 CPUs or less, we use the max. With more than 4 we use max - 1 to
preserve some bandwidth for the user's other programs.
(cherry picked from commit ea21122575)
