[Bug ada/81878] --disable-bootstrap --enable-languages=ada fails

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81878

--- Comment #24 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> This change broke the mingw-w64 GCC 8 bootstrap on Windows. The cleanup
> seems a bit too aggressive as now `-B ../..` is never passed down to xg++.

That shouldn't happen though, see comment #16.