Forgot to add dll_export.h when I was experimenting with compiling a DLL.

dll_export.h

+/* Copyright (c) 2006, NIF File Format Library and Tools
+All rights reserved.  Please see niflib.h for licence. */
+
+#ifndef _DLL_EXPORT_H_
+#define _DLL_EXPORT_H_
+
+#ifdef USE_NIFLIB_DLL
+	#ifdef _MSC_VER
+		#ifdef BUILDING_NIFLIB_DLL
+			#define NIFLIB_API __declspec(dllexport)
+		#else
+			#define NIFLIB_API __declspec(dllimport)
+		#endif
+		#define NIFLIB_HIDDEN
+	#else
+		#ifdef HAVE_GCCVISIBILITYPATCH
+			#define NIFLIB_API __attribute__ ((visibility("default")))
+			#define NIFLIB_HIDDEN __attribute__ ((visibility("hidden")))
+		#else
+			#define NIFLIB_API
+			#define NIFLIB_HIDDEN
+		#endif
+	#endif 
+#else
+	#define NIFLIB_API
+	#define NIFLIB_HIDDEN
+#endif
+
+#endif
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