Fix comment in c-decl.c

[Zack Weinberg]

Eric Christopher pointed out to me that I'd forgotten to say which
section of the C standard justified a particular bit of c-decl.c.

Tested with "make cc1", since this is entirely within a comment.

zw

        * c-decl.c (pushdecl): Clarify comment.

===================================================================
Index: c-decl.c
--- c-decl.c	22 Oct 2003 11:58:09 -0000	1.455
+++ c-decl.c	28 Oct 2003 01:04:12 -0000
@@ -1712,9 +1712,9 @@ pushdecl (tree x)
       if (DECL_EXTERNAL (x) || scope == global_scope)
 	{
 	  /* Find and check against a previous, not-in-scope, external
-	     decl for this identifier.  (C99 s???: If two declarations
-	     with external linkage, referring to the same object, have
-	     incompatible types, the behavior is undefined).  */
+	     decl for this identifier.  (C99 6.2.7p2: All declarations
+	     that refer to the same object or function shall have
+	     compatible type; otherwise, the behavior is undefined.)  */
 	  tree ext = any_external_decl (name);
 	  if (ext)
 	    {