cmGlobalGenerator: Implement VS6 check without virtual method.

Don't require existence of a global generator for this check.

Source/cmGlobalGenerator.h

@@ -242,7 +242,7 @@ public:
 
 #if defined(_WIN32) && !defined(__CYGWIN__)
   /** Is this the Visual Studio 6 generator?  */
-  virtual bool IsForVS6() const { return false; }
+  bool IsForVS6() const { return this->GetName() == "Visual Studio 6"; }
 #endif
 
   ///! Find a target by name by searching the local generators.

Source/cmGlobalVisualStudio6Generator.h

@@ -85,8 +85,6 @@ public:
 
   virtual void FindMakeProgram(cmMakefile*);
 
-  virtual bool IsForVS6() const { return true; }
-
 protected:
   virtual void Generate();
   virtual const char* GetIDEVersion() { return "6.0"; }