assert, include path, and virtual ~ patches by Lukas Fittl

git-svn-id: svn://svn.code.sf.net/p/loki-lib/code/trunk@414 7ec92016-0320-0410-acc4-a06ded1c099a

include/loki/Visitor.h

@@ -131,6 +131,7 @@ namespace Loki
     public:
         typedef R ReturnType;
         typedef T ParamType;
+        virtual ~Visitor() {}
         virtual ReturnType Visit(ParamType&) = 0;
     };
 
@@ -140,6 +141,7 @@ namespace Loki
     public:
         typedef R ReturnType;
         typedef const T ParamType;
+        virtual ~Visitor() {}
         virtual ReturnType Visit(ParamType&) = 0;
     };