Adjust the two legacy stdc++ headers that still included it;
it seems they didn't themselves depend on any of the definitions it
contained. However, streambuf.h actually did, so put that definition in
libio.h so that it's always present.
(The stdc++ implementation will get the 'real' _G_config.h from
libroot glibc.)
Augustin Cavalier
aa3a5e7698