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

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

            Bug ID: 65704
           Summary: Provide portable versions of std::timed_mutex and
                    std::recursive_timed_mutex
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

Currently we don't define std::timed_mutex and std::recursive_timed_mutex
unless the POSIX Timeouts option (_POSIX_TIMEOUTS) is supported. At least
Darwin doesn't support that option.

We should provide portable versions based on a std::mutex and
std::condition_variable::wait_until().