Make cmLocalGenerator::ConvertToLinkReference virtual

This provides a mechanism for the local generator to override how
library search paths are generated.

Source/cmLocalGenerator.h

@@ -369,7 +369,7 @@ protected:
   std::string FindRelativePathTopBinary();
   void SetupPathConversions();
 
-  std::string ConvertToLinkReference(std::string const& lib);
+  virtual std::string ConvertToLinkReference(std::string const& lib);
 
   /** Check whether the native build system supports the given
       definition.  Issues a warning.  */