[PATCH] gcc parallel make check

[VandeVondele Joost]

>> > These numbers are useful to try and ensure the overhead (scaling factor) is reasonable, thanks.
>>
>> A nice improvement indeed.  The patched result is 15 times faster
>> than the serial unpatched run.  So there is room for improvement
>
> Note, the box used was oldish AMD 16-core, no ht, box, haven't tried it on anything

on a 32 core box, no ht, I see these timings:

time make -j32 -k check >& log.check32 ; time make -j8 -k check >& log.check8

real	18m14.562s
user	260m21.578s
sys	264m26.042s

real	41m33.210s
user	233m4.563s
sys	72m11.429s

so it is not quite reaching the ideal 4x speedup. Counting the number of 'expect' processes they are nicely at around 32 and 8 for the full test, with only a very short tail near the end. So, there might be some overhead somewhere. Total user time is similar, but time in sys goes up.