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

YSFConvolution.cpp

@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2009-2016,2021 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2009-2016,2021,2023 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
@@ -18,6 +18,8 @@
 
 #include "YSFConvolution.h"
 
+#if defined(USE_YSF)
+
 #include <cstdio>
 #include <cassert>
 #include <cstring>
@@ -148,3 +150,6 @@ void CYSFConvolution::encode(const unsigned char* in, unsigned char* out, unsign
 		k++;
 	}
 }
+
+#endif
+