Use __POSIX_VISIBLE not __POSIX_C_SOURCE to bracket the definitions

of wcscasecmp() and wcsncasecmp().  Noticed by naddy@ OK espie@

src/include/wchar.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: wchar.h,v 1.16 2011/05/28 15:16:46 espie Exp $	*/
+/*	$OpenBSD: wchar.h,v 1.17 2011/06/01 16:39:07 millert Exp $	*/
 /*	$NetBSD: wchar.h,v 1.16 2003/03/07 07:11:35 tshiozak Exp $	*/
 
 /*-
@@ -145,7 +145,7 @@ long int wcstol(const wchar_t * __restrict, wchar_t ** __restrict, int base);
 unsigned long int wcstoul(const wchar_t * __restrict, wchar_t ** __restrict,
 		int base);
 
-#if __POSIX_C_SOURCE >= 200809L
+#if __POSIX_VISIBLE >= 200809
 int wcscasecmp(const wchar_t *, const wchar_t *);
 int wcsncasecmp(const wchar_t *, const wchar_t *, size_t);
 #endif