SCons: Default `num_jobs` to max CPUs minus 1 if not specified

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.