Add assert to enable compilation with libcxx + gcc (#947)

include/yaml-cpp/node/iterator.h

@@ -15,6 +15,9 @@
 #include <utility>
 #include <vector>
 
+// Assert in place so gcc + libc++ combination properly builds
+static_assert(std::is_constructible<YAML::Node, const YAML::Node&>::value, "Node must be copy constructable");
+
 namespace YAML {
 namespace detail {
 struct iterator_value : public Node, std::pair<Node, Node> {