support infinite and default 'parallel jobs' in build steps

in the 'make' build steps of my generic project it is apparently impossible to specify that the job count should be left alone. this interferes with my build tool wrapper, which determines this automatically, unless specified at the command line.

when this is fixed, it would probably also make sense to add an option for 'unlimited', i.e., a plain -j.

on a (possibly) unrelated note, the "Override MAKEFLAGS" checkbox next to the job spinbox has no tooltip that would suggest what it really does or is supposed to be good for.