Merge pull request dmlc#38 from dmlc/concurrency-fix-for-gcc-4.8

[concurrency-fix-for-gcc-4.8] member initialization doesn't work on G++ 4.8

include/dmlc/concurrency.h

@@ -46,7 +46,7 @@ class Spinlock {
 template <typename T>
 class ConcurrentBlockingQueue {
  public:
-  ConcurrentBlockingQueue() = default;
+  ConcurrentBlockingQueue();
   ~ConcurrentBlockingQueue() = default;
   /*!
    * \brief Push element into the queue.
@@ -83,7 +83,7 @@ class ConcurrentBlockingQueue {
  private:
   std::mutex mutex_;
   std::condition_variable cv_;
-  std::atomic<bool> exit_now_{false};
+  std::atomic<bool> exit_now_;
   std::list<T> queue_;
   /*!
    * \brief Disable copy and move.
@@ -100,6 +100,9 @@ inline void Spinlock::unlock() noexcept {
   lock_.clear(std::memory_order_release);
 }
 
+template <typename T>
+ConcurrentBlockingQueue<T>::ConcurrentBlockingQueue() : exit_now_{false} {}
+
 template <typename T>
 template <typename E>
 void ConcurrentBlockingQueue<T>::Push(E&& e) {