Make all of the rest of the code base be conditionally compiled.

NXDNCRC.h

@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2018 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2018,2023 by Jonathan Naylor G4KLX
  *
  *   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -19,6 +19,10 @@
 #if !defined(NXDNCRC_H)
 #define	NXDNCRC_H
 
+#include "Defines.h"
+
+#if defined(USE_NXDN)
+
 #include <cstdint>
 
 class CNXDNCRC
@@ -40,3 +44,6 @@ private:
 };
 
 #endif
+
+#endif
+