Fixed beta_version declaration.

src/sbbs3/atcodes.cpp

@@ -38,7 +38,7 @@
 #include "sbbs.h"
 #include "cmdshell.h"
 
-extern const char* beta_version;
+extern "C" const char* beta_version;
 
 /****************************************************************************/
 /* Returns 0 if invalid @ code. Returns length of @ code if valid.          */