PORT: Use C99 constructs for a portable version of struct functor

Structs may use ``type name[]`` as last field for a dynamic array.

src/pl-incl.h

@@ -1602,7 +1602,7 @@ struct mark
 
 struct functor
 { word		definition;	/* Tagged definition pointer */
-  word		arguments[1];	/* arguments vector */
+  word		arguments[];	/* arguments vector */
 };
 
 struct clause_bucket