Forward-declare types only used in DataIdentity.cpp

library/DataIdentity.cpp

@@ -1,11 +1,13 @@
 #include <stddef.h>
 
+#include <condition_variable>
 #include <fstream>
 #include <mutex>
 #include <string>
 #include <vector>
 
 #include "DataFuncs.h"
+#include "DataIdentity.h"
 
 namespace df {
 #define NUMBER_IDENTITY_TRAITS(category, type, name) \

library/include/DataIdentity.h

@@ -24,16 +24,19 @@ distribution.
 
 #pragma once
 
-#include <condition_variable>
 #include <deque>
 #include <map>
-#include <mutex>
 #include <sstream>
 #include <string>
 #include <vector>
 
 #include "DataDefs.h"
 
+namespace std {
+    class condition_variable;
+    class mutex;
+};
+
 /*
  * Definitions of DFHack namespace structs used by generated headers.
  */