boost::none_t no longer convertible from literal 0 in 1.58

Instead we use boost::none, which has been documented since boost 1.34.0.

src/account.h

@@ -261,11 +261,7 @@ public:
   mutable optional<xdata_t> xdata_;
 
   bool has_xdata() const {
-#if BOOST_VERSION >= 105600
-    return xdata_ != NULL;
-#else
-    return xdata_;
-#endif
+    return xdata_ != boost::none;
   }
   void clear_xdata();
   xdata_t& xdata() {

src/item.h

@@ -174,11 +174,7 @@ public:
   static bool use_aux_date;
 
   virtual bool has_date() const {
-#if BOOST_VERSION >= 105600
-    return _date != NULL;
-#else
-    return _date;
-#endif
+    return _date != boost::none;
   }
 
   virtual date_t date() const {

src/parser.h

@@ -118,7 +118,7 @@ public:
 
   ptr_op_t parse(std::istream&           in,
                  const parse_flags_t&    flags           = PARSE_DEFAULT,
-                 const optional<string>& original_string = NULL);
+                 const optional<string>& original_string = boost::none);
 };
 
 } // namespace ledger

src/post.h

@@ -205,11 +205,7 @@ public:
   mutable optional<xdata_t> xdata_;
 
   bool has_xdata() const {
-#if BOOST_VERSION >= 105600
-    return xdata_ != NULL;
-#else
-    return xdata_;
-#endif
+    return xdata_ != boost::none;
   }
   void clear_xdata() {
     xdata_ = none;

src/times.h

@@ -500,11 +500,7 @@ public:
   void   stabilize(const optional<date_t>& date = none);
 
   bool   is_valid() const {
-#if BOOST_VERSION >= 105600
-    return start != NULL;
-#else
-    return start;
-#endif
+    return start != boost::none;
   }
 
   /** Find the current or next period containing date.  Returns false if