[Bug c/106672] support Apple's old __private_extern__ keyword

[egallager at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106672

--- Comment #2 from Eric Gallager <egallager at gcc dot gnu.org> ---
Well, at the very least GCC could print a nicer error message (and possibly
suggest a fix-it hint for it, too)