[Bug bootstrap/86518] Strengthen bootstrap comparison by not enabling warnings at stage3

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86518

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I wouldn't be surprised if the build fails with -Wnarrowing (but maybe that
should be fixed anyway, so we can build with non-GNU compilers that default to
C++11 or later).