silence msvc-14.0 warning

Author: grisumbras

include/boost/json/detail/string_impl.hpp

@@ -141,7 +141,13 @@ class string_impl
         std::random_access_iterator_tag)
         : string_impl(last - first, sp)
     {
-        std::copy(first, last, data());
+        char* out = data();
+#if defined(_MSC_VER) && _MSC_VER <= 1900
+        while( first != last )
+            *out++ = *first++;
+#else
+        std::copy(first, last, out);
+#endif
     }
 
     template<class InputIt>

include/boost/json/impl/string.hpp

@@ -194,10 +194,16 @@ append(
     InputIt last,
     std::random_access_iterator_tag)
 {
+
     auto const n = static_cast<
         size_type>(last - first);
-    std::copy(first, last,
-        impl_.append(n, sp_));
+    char* out = impl_.append(n, sp_);
+#if defined(_MSC_VER) && _MSC_VER <= 1900
+    while( first != last )
+        *out++ = *first++;
+#else
+    std::copy(first, last, out);
+#endif
 }
 
 template<class InputIt>