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/Transaction.h

@@ -94,6 +94,10 @@ typedef struct _TransactionClass TransactionClass;
 #include "gnc-engine.h"
 #include "Split.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* --- type macros --- */
 #define GNC_TYPE_TRANSACTION            (gnc_transaction_get_type ())
 #define GNC_TRANSACTION(o)              \
@@ -789,6 +793,10 @@ void xaccTransDump (const Transaction *trans, const char *tag);
 /** \deprecated */
 #define xaccTransReturnGUID(X) (X ? *(qof_entity_get_guid(QOF_INSTANCE(X))) : *(guid_null()))
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
 #endif /* XACC_TRANSACTION_H */
 /** @} */
 /** @} */