fix up static, so works as DLL (#559)

* fix up static, so works as DLL

include/yaml-cpp/node/detail/node_data.h

@@ -81,7 +81,7 @@ class YAML_CPP_API node_data {
                     shared_memory_holder pMemory);
 
  public:
-  static std::string empty_scalar;
+  static const std::string& empty_scalar();
 
  private:
   void compute_seq_size() const;

include/yaml-cpp/node/impl.h

@@ -156,13 +156,13 @@ inline T Node::as(const S& fallback) const {
 inline const std::string& Node::Scalar() const {
   if (!m_isValid)
     throw InvalidNode();
-  return m_pNode ? m_pNode->scalar() : detail::node_data::empty_scalar;
+  return m_pNode ? m_pNode->scalar() : detail::node_data::empty_scalar();
 }
 
 inline const std::string& Node::Tag() const {
   if (!m_isValid)
     throw InvalidNode();
-  return m_pNode ? m_pNode->tag() : detail::node_data::empty_scalar;
+  return m_pNode ? m_pNode->tag() : detail::node_data::empty_scalar();
 }
 
 inline void Node::SetTag(const std::string& tag) {

src/node_data.cpp

@@ -13,7 +13,10 @@
 namespace YAML {
 namespace detail {
 
-std::string node_data::empty_scalar;
+const std::string& node_data::empty_scalar() {
+    static const std::string svalue;
+    return svalue;
+}
 
 node_data::node_data()
     : m_isDefined(false),