make -jN loops on Windows

[Simon Peyton-Jones]

I have a nice new 64-bit Windows 8 laptop.  But when I say
                make -j2
then it launches a couple processes (which I think complete fine) and then locks up with 'make' consuming 25-35% of a CPU, but nothing happening.  No further activity for 30 mins.
This is msys 'make', version 3.81, which worked fine on my old (32-bit) laptop.
Things work ok without -j.  But validate uses -j (how to disable that?).
Does anyone have a clue about how I might find out what is going on?  I don't think msys has 'strace'.
Thanks
Simon

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