libstdc++ headers and system headers

[Neil Booth <neil at daikokuya dot demon dot co dot uk>]

Compiling a file containing merely

#include <iostream.h>

with

g++ -E test.c -o test.ii -I /usr/local/include/g++-v3/

and then re-compiling the preprocessed file gives a bunch of warnings
about header flags changing.  I've just realized why: various headers
in the bits/ directory have

#pragma GCC system_header

in them.  Since the command line has "-I", the directories are not
considered system directories, so the preprocessed output has no "3"
flag when entering the file, but when returning to the file from a nested
include, it has since become a system header owing to the pragma and the
"3" flag magically appears.  CPP complains when re-preprocessing and
the flags don't match.

I don't know why libstdc++ uses the pragma, but that's another issue.

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?

Neil.