Fix -Wundef warning.  Upstream removed the __ASSUME_ARG_MAX_STACK_BASE
preprocessor conditional as part of this commit:

commit 637461d96563f3b7405df5b124ccfe843e537374
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon May 12 22:48:25 2014 +0000

    Clean up kernel version conditionals for pre-2.6.32 kernels.
    
    This patch does some initial cleanup, following the move to 2.6.32
    minimum kernel version, by removing __LINUX_KERNEL_VERSION
    conditionals that are now always-true or always-false.  In the case of
    __ASSUME_ARG_MAX_STACK_BASED, where the conditional used a kernel
    version that was itself in a macro, the associated sysconf.c code is
    also cleaned up and __ASSUME_ARG_MAX_STACK_BASED removed completely.

diff --git a/sysdeps/unix/sysv/linux/sysconf.c b/sysdeps/unix/sysv/linux/sysconf.c
index 47ce5ef07cce7888..05cbb0879e9748d5 100644
--- a/sysdeps/unix/sysv/linux/sysconf.c
+++ b/sysdeps/unix/sysv/linux/sysconf.c
@@ -88,7 +88,7 @@ __sysconf (int name)
       return HAS_CPUCLOCK (name);
 
     case _SC_ARG_MAX:
-#if !__ASSUME_ARG_MAX_STACK_BASED
+#ifndef __ASSUME_ARG_MAX_STACK_BASED
       /* Determine whether this is a kernel with an argument limit
 	 determined by the stack size.  */
       if (GLRO(dl_discover_osversion) ()