Backport of this upstream commit:

commit 7f9d00341057eb80b43fa83956b8d7798b4dabea
Author: Roland McGrath <roland@hack.frob.com>
Date:   Tue Aug 27 11:09:33 2013 -0700

    Clean up h_errno declaration to use __thread unconditionally.

Adjusted for the IS_IN changes, which were applied upstream after this
commit (and downstream before its backport).

diff --git a/include/netdb.h b/include/netdb.h
index 3b24747fd5df6562..b6d7b90bbf8abd2e 100644
--- a/include/netdb.h
+++ b/include/netdb.h
@@ -5,17 +5,12 @@
 /* Macros for accessing h_errno from inside libc.  */
 # if IS_IN_LIB
 #  undef  h_errno
-#  ifdef _LIBC_REENTRANT
-#   include <tls.h>
-#   if IS_IN (libc)
-#    define h_errno __libc_h_errno
-#   else
-#    define h_errno h_errno	/* For #ifndef h_errno tests.  */
-#   endif
-extern __thread int h_errno attribute_tls_model_ie;
+#  if IS_IN (libc)
+#   define h_errno __libc_h_errno
 #  else
-extern int h_errno;
-#  endif	/* _LIBC_REENTRANT */
+#   define h_errno h_errno	/* For #ifndef h_errno tests.  */
+#  endif
+extern __thread int h_errno attribute_tls_model_ie;
 # endif /* IS_IN_LIB */
 # define __set_h_errno(x) (h_errno = (x))