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.