OK, go for it. I just looked and internal seemed more stricter, however
hidden is also fine.

> Hello,
> I am running into a problem with our iOS builds. The updated libraries
> use -fvisibility=internal as a compiler option, but the old clang
> version in Xcode doesn't understand this. It does understand
> -fvisiblity=hidden, however, which does basically the same thing.
>
> While internal visibility does give the compiler more optimization
> options than hidden visiblity, it is also a bit more dangerous. In
> particular, it can silently break function pointers (but only if you
> are unlucky). For full correctness, we would have to put a visibility
> directive in front of every internal function that would be used as a
> callback from another library.
>
> So, I propose we just move everything to -fvisiblity=hidden,
> un-breaking the iOS build and avoiding the potential for subtle bugs.
>
> -William
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