Adds support the variable rule

Issue #1016

src/actions/msg.cc

@@ -53,11 +53,16 @@ bool Msg::evaluate(Rule *rule, Transaction *transaction) {
     transaction->debug(9, "Saving msg: " + msg);
 #endif
 
-    rule->m_log_message = msg;
+    rule->m_log_message = data(transaction);
 
     return true;
 }
 
 
+std::string Msg::data(Transaction *transaction) {
+    return MacroExpansion::expand(m_parser_payload, transaction);
+}
+
+
 }  // namespace actions
 }  // namespace modsecurity