Account.c to Account.cpp

Since Account.c is now Account.cpp, the function signatures look a bit
different internally. The tests rely on function signatures in error
messages. Instead of trying to figure out what the exact
function signature might be, I use a substring matching strategy to
ensure that the correct error was issued.

libgnucash/engine/policy.h

@@ -37,6 +37,10 @@
 #ifndef XACC_POLICY_H
 #define XACC_POLICY_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct gncpolicy_s GNCPolicy;
 
 /** Valid Policy List
@@ -83,6 +87,10 @@ GNCPolicy *xaccGetFIFOPolicy (void);
  */
 GNCPolicy *xaccGetLIFOPolicy (void);
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
 #endif /* XACC_POLICY_H */
 /** @} */
 /** @} */