https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100655
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- It's not glibc-specific though, it's going to be in the next POSIX standard and other C libraries. It's a bit of a hack, but you could include any libstdc++ header and then check _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT (because libstdc++ already has autoconf checks for it).
