ENH: Make IF command scriptable

Source/cmIfCommand.h

@@ -87,6 +87,11 @@ public:
    */
   virtual bool IsInherited() {return true;}
 
+  /**
+   * This determines if the command is invoked when in script mode.
+   */
+  virtual bool IsScriptable() { return true; }
+
   /**
    * More documentation.
    */