diff --git a/ares/m4/cares-reentrant.m4 b/ares/m4/cares-reentrant.m4
index 8962da47c71098ec9c28bde2f136fc264274d75d..5793b779144b7281059abb044c431a39a38cef1b 100644
--- a/ares/m4/cares-reentrant.m4
+++ b/ares/m4/cares-reentrant.m4
@@ -1,7 +1,7 @@
 #***************************************************************************
 # $Id$
 #
-# Copyright (C) 2008 by Daniel Stenberg et al
+# Copyright (C) 2008 - 2009 by Daniel Stenberg et al
 #
 # Permission to use, copy, modify, and distribute this software and its
 # documentation for any purpose and without fee is hereby granted, provided
@@ -16,7 +16,7 @@
 #***************************************************************************
 
 # File version for 'aclocal' use. Keep it a single number.
-# serial 3
+# serial 4
 
 dnl Note 1
 dnl ------
@@ -25,6 +25,58 @@ dnl conditionally include header files. These macros are used early in the
 dnl configure process much before header file availability is known.
 
 
+dnl CARES_CHECK_NEED_REENTRANT_ERRNO
+dnl -------------------------------------------------
+dnl Checks if the preprocessor _REENTRANT definition
+dnl makes errno available as a preprocessor macro.
+
+AC_DEFUN([CARES_CHECK_NEED_REENTRANT_ERRNO], [
+  AC_COMPILE_IFELSE([
+    AC_LANG_PROGRAM([[
+#include <errno.h>
+    ]],[[
+      if(0 != errno)
+        return 1;
+    ]])
+  ],[
+    tmp_errno="yes"
+  ],[
+    tmp_errno="no"
+  ])
+  if test "$tmp_errno" = "yes"; then
+    AC_COMPILE_IFELSE([
+      AC_LANG_PROGRAM([[
+#include <errno.h>
+      ]],[[
+#ifdef errno
+        int dummy=1;
+#else
+        force compilation error
+#endif
+      ]])
+    ],[
+      tmp_errno="errno_macro_defined"
+    ],[
+      AC_COMPILE_IFELSE([
+        AC_LANG_PROGRAM([[
+#define _REENTRANT
+#include <errno.h>
+        ]],[[
+#ifdef errno
+          int dummy=1;
+#else
+          force compilation error
+#endif
+        ]])
+      ],[
+        tmp_errno="errno_macro_needs_reentrant"
+        tmp_need_reentrant="yes"
+      ])
+    ])
+  fi
+])
+
+
 dnl CARES_CHECK_NEED_REENTRANT_GMTIME_R
 dnl -------------------------------------------------
 dnl Checks if the preprocessor _REENTRANT definition
@@ -437,6 +489,9 @@ AC_DEFUN([CARES_CONFIGURE_REENTRANT], [
   if test "$tmp_reentrant_initially_defined" = "no"; then
     AC_MSG_CHECKING([if _REENTRANT is actually needed])
     CARES_CHECK_NEED_REENTRANT_SYSTEM
+    if test "$tmp_need_reentrant" = "no"; then
+      CARES_CHECK_NEED_REENTRANT_ERRNO
+    fi
     if test "$tmp_need_reentrant" = "no"; then
       CARES_CHECK_NEED_REENTRANT_FUNCTIONS_R
     fi
diff --git a/m4/curl-reentrant.m4 b/m4/curl-reentrant.m4
index 9b6c9663fc9aff928f68dc9597a584bff23e6525..a2aa5a96ebde39be2fe83648ff615dc6e45f188d 100644
--- a/m4/curl-reentrant.m4
+++ b/m4/curl-reentrant.m4
@@ -5,7 +5,7 @@
 #                            | (__| |_| |  _ <| |___
 #                             \___|\___/|_| \_\_____|
 #
-# Copyright (C) 1998 - 2008, Daniel Stenberg, <daniel@haxx.se>, et al.
+# Copyright (C) 1998 - 2009, Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # This software is licensed as described in the file COPYING, which
 # you should have received as part of this distribution. The terms
@@ -22,7 +22,7 @@
 #***************************************************************************
 
 # File version for 'aclocal' use. Keep it a single number.
-# serial 3
+# serial 6
 
 dnl Note 1
 dnl ------
@@ -31,6 +31,58 @@ dnl conditionally include header files. These macros are used early in the
 dnl configure process much before header file availability is known.
 
 
+dnl CURL_CHECK_NEED_REENTRANT_ERRNO
+dnl -------------------------------------------------
+dnl Checks if the preprocessor _REENTRANT definition
+dnl makes errno available as a preprocessor macro.
+
+AC_DEFUN([CURL_CHECK_NEED_REENTRANT_ERRNO], [
+  AC_COMPILE_IFELSE([
+    AC_LANG_PROGRAM([[
+#include <errno.h>
+    ]],[[
+      if(0 != errno)
+        return 1;
+    ]])
+  ],[
+    tmp_errno="yes"
+  ],[
+    tmp_errno="no"
+  ])
+  if test "$tmp_errno" = "yes"; then
+    AC_COMPILE_IFELSE([
+      AC_LANG_PROGRAM([[
+#include <errno.h>
+      ]],[[
+#ifdef errno
+        int dummy=1;
+#else
+        force compilation error
+#endif
+      ]])
+    ],[
+      tmp_errno="errno_macro_defined"
+    ],[
+      AC_COMPILE_IFELSE([
+        AC_LANG_PROGRAM([[
+#define _REENTRANT
+#include <errno.h>
+        ]],[[
+#ifdef errno
+          int dummy=1;
+#else
+          force compilation error
+#endif
+        ]])
+      ],[
+        tmp_errno="errno_macro_needs_reentrant"
+        tmp_need_reentrant="yes"
+      ])
+    ])
+  fi
+])
+
+
 dnl CURL_CHECK_NEED_REENTRANT_GMTIME_R
 dnl -------------------------------------------------
 dnl Checks if the preprocessor _REENTRANT definition
@@ -443,6 +495,9 @@ AC_DEFUN([CURL_CONFIGURE_REENTRANT], [
   if test "$tmp_reentrant_initially_defined" = "no"; then
     AC_MSG_CHECKING([if _REENTRANT is actually needed])
     CURL_CHECK_NEED_REENTRANT_SYSTEM
+    if test "$tmp_need_reentrant" = "no"; then
+      CURL_CHECK_NEED_REENTRANT_ERRNO
+    fi
     if test "$tmp_need_reentrant" = "no"; then
       CURL_CHECK_NEED_REENTRANT_FUNCTIONS_R
     fi