[Bug c++/87380] Explicit instantations should use weak symbols on darwin

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87380

--- Comment #8 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Iain Sandoe from comment #7)
> it's been broken a looooong time - even apple-gcc-4.2.1 is broken

actually, that's not true; it puts the symbol in common to merge this.