libstdc++ headers and system headers

[Jason Merrill]

>>>>> "Neil" == Neil Booth <neil@daikokuya.demon.co.uk> writes:

> The bug is that CPP is not creating a file_change callback for #pragma,
> but I think it should do (like it would for #include or for #line).
> This callback would then result in the correct flags output, and the
> warnings would disappear.  It would also have the benefit that the front
> ends know about #pragma system_header; at present they are not informed
> of the change in state.

> Do you agree this is the correct fix, Jason?

Sounds good to me, thanks!

Jason