We've reached the point where it's no longer worth carrying around extra
patch baggage to help iperf 2.0.2 build on newer libcs and compilers.
One reason for that is, when iperf modernized to handle gcc6, they did
it at the same time as a nontrivial reorganization.  So we cannot just
trivially lift their bits, at all.

Second problem: iperf uses a binary packet header.  Our patch used
header bitspace that modern iperfs have since begun using for an
extended header.  So, the 2.0.10 patch in this commit attempts to
maintain binary compat with our hacked 2.0.2 iperf -- but throws out
compat with modern, regular iperfs.

Anyway, this commit just makes the iperf version choice optional, and
it's only 2.0.10 on Alpine Linux; all other build envs will still try to
build 2.0.2 (unless they are ISBRROKEN, of course).

(I know Mike^H^H^H^Hsomeone plans to revisit whether or not our patch is
still even necessary (my bet is that it is, after reading the code), but
I need this commit to get the clientside going on Alpine Linux.)