Link test/regress with event_core/event_extra over event
Due to regress linked with event and event_core (both of them includes
evthread.c) there will be two different evthread_id_fn_ variables under
mingw64:
evthread_id_fn_: &0x5294f20a8
evthread_id_fn_: &0x4ba0030a8
And because of this evthread_use_pthreads() can/will set one copy of
variables while evthread*() functions will access another, which will
break a lot of things (for example main/del_notify test).