[Bug libstdc++/65704] Provide portable versions of std::timed_mutex and std::recursive_timed_mutex

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65704

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 35273
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35273&action=edit
something like this