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/Account.h

@@ -48,6 +48,9 @@
 #include "gnc-engine.h"
 #include "policy.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 typedef gnc_numeric (*xaccGetBalanceFn)( const Account *account );
 
 typedef gnc_numeric (*xaccGetBalanceInCurrencyFn) (
@@ -1517,6 +1520,10 @@ const char * dxaccAccountGetQuoteTZ (const Account *account);
  * in the gnome-search parameter list.  Be careful when you use this. */
 #define ACCOUNT_MATCH_ALL_TYPE	"account-match-all"
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
 #endif /* XACC_ACCOUNT_H */
 /** @} */
 /** @} */