`make -O` when non-parallel

[Stefan Monnier]

I like the fact that `make -O` gives me output that's always meaningful,
but it's sometimes annoying how it's sitting idly while a command is
spewing output.  To some extent it's unavoidable, but I wonder if we
could reduce the inconvenience by making such that Make emits the
subprocess's output without delay *if* that's the only subprocess active
(i.e. we're not waiting to see which process ends first before emitting
its output).


        Stefan