Fix -Wundef warnings.  Upstream addressed this as part of this commit:

commit b100f9aa77027e4371154820ea8a0353bfd13223
Author: Will Newton <will.newton@linaro.org>
Date:   Mon Jun 23 16:53:30 2014 +0100

    malloc/obstack: Merge from gnulib

diff --git a/malloc/obstack.c b/malloc/obstack.c
index e3ce53f987727eca..2b103727e010b417 100644
--- a/malloc/obstack.c
+++ b/malloc/obstack.c
@@ -56,10 +56,10 @@
 #ifndef ELIDE_CODE
 
 
-# if HAVE_INTTYPES_H
+# if _LIBC || HAVE_INTTYPES_H
 #  include <inttypes.h>
 # endif
-# if HAVE_STDINT_H || defined _LIBC
+# if _LIBC || HAVE_STDINT_H
 #  include <stdint.h>
 # endif
 
diff --git a/malloc/obstack.h b/malloc/obstack.h
index a791823cf3f18348..69e92d1146b02eb1 100644
--- a/malloc/obstack.h
+++ b/malloc/obstack.h
@@ -250,13 +250,6 @@ extern int obstack_exit_failure;
 #define obstack_memory_used(h) _obstack_memory_used (h)
 
 #if defined __GNUC__
-/* NextStep 2.0 cc is really gcc 1.93 but it defines __GNUC__ = 2 and
-   does not implement __extension__.  But that compiler doesn't define
-   __GNUC_MINOR__.  */
-# if __GNUC__ < 2 || (__NeXT__ && !__GNUC_MINOR__)
-#  define __extension__
-# endif
-
 /* For GNU C, if not -traditional,
    we can define these macros to compute all args only once
    without using a global variable.