[Bug libstdc++/69386] [6 regression] r232586 breaks mingw-w64 bootstrap

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69386

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 37407
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37407&action=edit
Set language linkage for C++ headers

Does this fix it?