[Bug libstdc++/58929] condition_variable does not wait without -pthread

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58929

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't think -pthread sets _REENTRANT for all targets, although it might do
for all those where we actually support std::condition_variable.