g++ does not suppress warnings even isystem flag is set

[Dmitry K]

Hello. We use g++ to compile our big project and we use many 3rd party 
libraries and frameworks inside(like Qt or Poco). Some of these 
frameworks generate many warnings, so we use flag isystem to mark them 
as system to suppress 3dparty warnings. But it seems not all warnings 
suppressing by this flag. As result we have dozens 3dparty warnings in 
our project - they very very garbaging  build logs.

I found similar issue 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100570 but it's still even 
in UNCONFIRMED status - my opinion all just forgot about this issue.