Bring iperf-2.0.2 up to speed for gccs up through 8.x.
Unfortunately, I didn't notice the first time, but the include causing all the problems was an apparently unnecessary math.h. Simply removing it fixed all the odd libstdc++ errors; trying to fix them all up and leaving it in was much more complicated. This works on gccs 4, 6, 7, 8. This is a good outcome because the iperf-2.0.10 patch is still subtly wrong in some important way; bandwidths are inconsistent or sometimes consistently asymmetric (with no good reason). So some parameter must still not be being exchanged between client and server.
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