Setting 'make -jN' in makeprg

A new tip suggests a vimrc command to set makeprg so it will
automatically detect the number of processors and adjust the
'-j' make option. This is out of my league (although I have seen
a friend build the Linux kernel using -j16, although I think he
gave that up because of the confusing error messages), but I
suggested the following replacement:

let &makeprg = 'make -j'.system('echo -n "$(echo $(grep -c ^processor /proc/cpuinfo)+1 | bc)"')

Any thoughts on whether that is correct and useful? The tip is:
http://vim.wikia.com/wiki/Auto-detect_number_of_cores_for_parallel_built

John

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