Add option for parallel builds to make step

On Windows, Qt Creator uses "jom" by default to build in parallel, and has an option to enable/disable this.

On Mac, Qt Creator does not attempt parallel builds and does not have any options to enable parallel builds.

When using a "Make"-based toolchain, it is possible to force a parallel build by adding the command-line argument "-j%NUMBER%" to the Make arguments.
An appropriate value for %NUMBER% can be determined by the user via the Terminal command "sysctl -n hw.ncpu" to find the number of logical CPU cores.

This is very obtuse and should not be necessary.
Additionally, this setting is not possible to set via qmake and so this affects all users when loading an existing project.

This is probably wasting thousands of user hours every day.

Qt Creator on Mac should use parallel make by default, with an option to override it.