Suppress compiler warnings in readfuncs.c.

Commit 7357558fc8 introduced "(void) token;"
into the READ_TEMP_LOCALS() macro, to suppress complaints from gcc 4.6
when the value of token was not used anywhere in a particular node-read
function.  However, this just moved the warning around: inspection of
buildfarm results shows that some compilers are now complaining that token
is being read before it's set.  Revert the READ_TEMP_LOCALS() macro change
and instead put "(void) token;" into READ_NODE_FIELD(), which is the
principal culprit for cases where the warning might occur.  In principle we
might need the same in READ_BITMAPSET_FIELD() and/or READ_LOCATION_FIELD(),
but it seems unlikely that a node would consist only of such fields, so
I'll leave them alone for now.

src/backend/nodes/readfuncs.c

@@ -48,8 +48,7 @@
 /* And a few guys need only the pg_strtok support fields */
 #define READ_TEMP_LOCALS()	\
 	char	   *token;		\
-	int			length;		\
-	(void) token				/* possibly unused */
+	int			length
 
 /* ... but most need both */
 #define READ_LOCALS(nodeTypeName)			\
@@ -113,6 +112,7 @@
 /* Read a Node field */
 #define READ_NODE_FIELD(fldname) \
 	token = pg_strtok(&length);		/* skip :fldname */ \
+	(void) token;				/* in case not used elsewhere */ \
 	local_node->fldname = nodeRead(NULL, 0)
 
 /* Read a bitmapset field */