[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 #5 from Iain Sandoe <iains at gcc dot gnu.org> ---
fudging the static member to be weak, and rebuilding the lib - the test
completes w/out throwing.