Remove use of deprecated allocator rebind in cpp-btree

src/3rdparty/cpp-btree/btree.h

@@ -917,8 +917,7 @@ class btree : public Params::key_compare {
   typedef std::reverse_iterator<iterator> reverse_iterator;
 
   typedef typename Params::allocator_type allocator_type;
-  typedef typename allocator_type::template rebind<char>::other
-    internal_allocator_type;
+  using internal_allocator_type = typename std::allocator_traits<allocator_type>::template rebind_alloc<char>;
 
  public:
   // Default constructor.