Have g++ define _FILE_OFFSET_BITS=64 on Solaris

[Rainer Orth]

Hi Jonathan,

> No objection to this patch, but I'll just note that we have
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81091 suggesting we
> should use LFS for libstdc++ unconditionally.

seems like a wise move to me.  The libstdc++.so ABI didn't change on
Solaris either (that possibility had caused concern for me initially);
didn't check libstdc++fs.a though.

	Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University