[Patch draft] Fix libstdc++/8610

[Paolo Carlini]

Andreas Jaeger wrote:

>For both you need to define -D_LARGEFILE_SOURCE64 on glibc, otherwise
>the header will not make the prototypes visible.  I don't think this
>is done yet and therefore I expect that you get warnings that there's
>no prototype for these functions...
>  
>
I see, in principle. However, for some reason,, we don't get any warning
at build time (-Wall), neither compiling actual code (try the attached
test64.cc -Wall -Wextra, f.i.). I don't know exactly /why/ at the moment,
I should, but this makes things much easier... ;)

Paolo.