No default conversion from std::ostringstream to char *.

Index: ddd/VSLDefList.C
--- ddd/VSLDefList.C.orig
+++ ddd/VSLDefList.C
@@ -60,7 +60,7 @@ const Box *VSLDefList::eval(Box *arg) const
     {
 	std::ostringstream s;
 	s << *arg;
-	VSLLib::eval_error("no suiting definition for " + f_name() + s);
+	VSLLib::eval_error("no suiting definition for " + f_name() + s.str().c_str());
     }
 
     return d ? d->eval(arg) : 0;