Use constrain_build macro to simplify parallel make handling
    
    With constrain_build it is possible to just say how much RAM per process
    / CPU core we need to build and it automatically calculates out the
    right -jXX number to use.
    
    I've used 2 GB per CPU as the initial setting, but if we run into issues
    with gcc running out of memory on e.g. s390x, the number can be easily
    increased (which then reduces parallelism on low memory builders).