This has been the official line since 2.17, which seems plenty long enough for a transition phase.
Based on a patch from Alexander Larsson. https://bugzilla.gnome.org/show_bug.cgi?id=651467
Add support for a mutex lock that consumes only one bit of storage inside of an integer on systems that support futexes. Futex is emulated (at a higher cost) on systems that don't have it -- but only in the contended case.
