fix the problem that butex_wake does not signal pending tasks (#13)

Author: MrGuin

src/bthread/condition_variable.cpp

@@ -43,6 +43,7 @@ extern "C" {
 
 extern int bthread_mutex_unlock(bthread_mutex_t*);
 extern int bthread_mutex_lock_contended(bthread_mutex_t*);
+extern void bthread_flush();
 
 int bthread_cond_init(bthread_cond_t* __restrict c,
                       const bthread_condattr_t*) {
@@ -67,6 +68,8 @@ int bthread_cond_signal(bthread_cond_t* c) {
     // don't touch ic any more
     bool no_signal = true;
     bthread::butex_wake(saved_seq, no_signal);
+    // flush unsignaled tasks manually
+    bthread_flush();
     return 0;
 }