[Bug bootstrap/63622] [5.0 Regression] Bootstrap fails on x86_64-apple-darwin1[34] after revision r216305

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63622

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to howarth from comment #14)
> which produces as set of warnings that don't appear in the build of
> libstdc++-v3/src/c++98/streambuf-inst.cc. These are...
> 
> ./../../../../gcc-5.0-20141105/libstdc++-v3/src/c++11/streambuf-inst.cc:63:1:
> warning: alias definitions not supported in Mach-O; ignored

I have no idea what that means or why building the file with -std=c++11 instead
of -std=c++98 causes it, and I don't have any way to check what is different on
darwin.