[v3] Suppress warning

[Gabriel Dos Reis]

Florian Weimer <fw@deneb.enyo.de> writes:

| * Paolo Bonzini:
| 
| >> To be clear: the warning itself has *always* been part of -Wall
| >> (just try yourself on 3.3, for instance): only, now triggers more
| >> often, also, many times, during the build of libstdc++-v3.
| >
| > I think that "while ((a = b))" is preferrable to "while (a = b)", but 
| > for other assignment operators the warning should not trigger.
| 
| I think it should also be activated for "/=" at least (because this is
| a comparison operator in other languages), maybe even for "<<=" and
| ">>=".

This is C++.

-- Gaby