[Bug libstdc++/55394] Using call_once without -lpthread compiles without warning

[slyfox at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55394

--- Comment #12 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
Aha, makes sense.

My hack did not survive bootstrap anyway as libgcc.a started referring
pthread_once() as well.