Avoid compiler warnings due to possibly unused variables

gcc 4.6 complains about these because of the new option
-Wunused-but-set-variable which comes in with -Wall, so cast them to
void, which avoids the warning.

src/backend/nodes/readfuncs.c

@@ -48,7 +48,8 @@
 /* And a few guys need only the pg_strtok support fields */
 #define READ_TEMP_LOCALS()	\
 	char	   *token;		\
-	int			length
+	int			length;		\
+	(void) token /* possibly unused */
 
 /* ... but most need both */
 #define READ_LOCALS(nodeTypeName)			\