Replace usleep with sleep

usleep was deprecated with POSIX 2008 and optionally unavailable with
uClibc-ng.

src/thread_base.cc

@@ -66,7 +66,7 @@ class lt_cacheline_aligned thread_queue_hack {
 
   thread_queue_hack() { std::memset(this, 0, sizeof(thread_queue_hack)); }
 
-  void     lock()   { while (!__sync_bool_compare_and_swap(&m_lock, 0, 1)) usleep(0); }
+  void     lock()   { while (!__sync_bool_compare_and_swap(&m_lock, 0, 1)) sleep(0); }
   void     unlock() { __sync_bool_compare_and_swap(&m_lock, 1, 0); }
 
   iterator begin() { return m_queue; }