rc-file: Add "num-cpus" option by default and use it in default setup.

This change introduces a "num-cpus" option that is inherently present in
the build context.

This permits config file reading code to refer to this option (due to an
existing kdesrc-build feature). So I also update the various available
methods of generating a default configuration to use this option instead
of hardcoding a -j value for make-options (or leaving it blank).

This should provide maximum performance for most users (who aren't using
or can't use the existing Ninja support, anyways), as long as they are
able to start from a generated configuration.

Users with existing configuration files would need to regenerate it or
edit it to add "make-options -j ${num-cpus}" (including separately to
their Qt5 options if applicable, as global options are ignored for
non-KDE modules).

See issue #39.