Fix -Wundef warnings.  Fixed upstream as part of this commit:

commit 1570a72bb8359c9a3a64c4e22d9ea47ce6fcff31
Author: Will Newton <will.newton@linaro.org>
Date:   Thu Jun 26 16:00:44 2014 +0100

    string/memchr.c: Merge from gnulib

diff --git a/string/memchr.c b/string/memchr.c
index 1684bfa56885e883..d1880cfbaed951a6 100644
--- a/string/memchr.c
+++ b/string/memchr.c
@@ -32,11 +32,11 @@
 # include <memcopy.h>
 #endif
 
-#if HAVE_STDLIB_H || defined _LIBC
+#if defined _LIBC || HAVE_STDLIB_H
 # include <stdlib.h>
 #endif
 
-#if HAVE_LIMITS_H || defined _LIBC
+#if defined _LIBC || HAVE_LIMITS_H
 # include <limits.h>
 #endif