[Bug libstdc++/64967] [5 Regression] Bootstrap fails due to errors in libstdc++ sources with `--enable-symvers=gnu-versioned-namespace'

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64967

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #2)
> Or drop the gnu-versioned-namespace support altogether.

I'd like to do that ... maybe for GCC 6 though. It seems a bit late now to
remove it without deprecation.

> Anyway, if you don't, bumping it to _8 and always using the new ABI sounds
> like a good plan to me.

That proved quite difficult. My plan for now is to simply force
--disable-libstdcxx-dual-abi when gnu-versioned-namespace is in use, so it
bootstraps and can be used, just without the new GCC 5 hotness.