[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 #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I wasn't actually testing the new code at all due to a cock up when trying to
undefine the macro that differs between darwin and other targets. Should be OK
now.