ENH: make properties a bit more formal with documentation and chaining

Source/cmCPluginAPI.h

@@ -170,6 +170,13 @@ typedef struct
   /* display status information */
   void  (CCONV *DisplaySatus) (void *info, const char *message);
 
+  /* new functions added after 2.4 */
+  void *(CCONV *CreateNewSourceFile) (void *mf);
+  void (CCONV *DefineSourceFileProperty) (void *mf, const char *name,
+                                          const char *briefDocs, 
+                                          const char *longDocs,
+                                          int chained);
+  
   /* this is the end of the C function stub API structure */
 } cmCAPI;