Allow multi-sign to be enabled at runtime:

This lets unit tests exercise multi-sign interfaces
without having to set RIPPLE_MULTI_SIGN_ENABLE.

src/ripple/protocol/STTx.h

@@ -123,7 +123,13 @@ public:
 
     void sign (RippleAddress const& private_key);
 
-    bool checkSign () const;
+    bool checkSign(bool allowMultiSign =
+#if RIPPLE_ENABLE_MULTI_SIGN
+        true
+#else
+        false
+#endif
+            ) const;
 
     bool isKnownGood () const
     {