Quite often I found my laptop lacking CPU power and memory when building a package in the background with -j$max_cpus and doing other stuff.

Having an option in bm to limit the number of cpus that can be used for parallel build would be nice.

It can already be achieved via

  bm --define '_smp_ncpus_max N'

where N is the wanted number.

But the macro's name is so hard to remember that a shorthand like `--cpus N` would be very welcome.