diff --git a/libsodium/configure.ac b/libsodium/configure.ac
index 6adb5ab8..b1c75231 100644
--- a/libsodium/configure.ac
+++ b/libsodium/configure.ac
@@ -137,8 +137,10 @@ AC_PROG_CC_C99
 AM_PROG_AS
 AC_USE_SYSTEM_EXTENSIONS
 
-AX_CHECK_COMPILE_FLAG([-D_FORTIFY_SOURCE=2],
-  [CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"])
+AC_CHECK_DEFINE([_FORTIFY_SOURCE], [], [
+  AX_CHECK_COMPILE_FLAG([-D_FORTIFY_SOURCE=2],
+    [CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"])
+])
 
 AX_CHECK_COMPILE_FLAG([-fvisibility=hidden],
   [CFLAGS="$CFLAGS -fvisibility=hidden"])
diff --git a/libsodium/m4/ax_check_define.m4 b/libsodium/m4/ax_check_define.m4
new file mode 100644
index 00000000..4bc69486
--- /dev/null
+++ b/libsodium/m4/ax_check_define.m4
@@ -0,0 +1,92 @@
+# ===========================================================================
+#      http://www.gnu.org/software/autoconf-archive/ax_check_define.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AC_CHECK_DEFINE([symbol], [ACTION-IF-FOUND], [ACTION-IF-NOT])
+#   AX_CHECK_DEFINE([includes],[symbol], [ACTION-IF-FOUND], [ACTION-IF-NOT])
+#
+# DESCRIPTION
+#
+#   Complements AC_CHECK_FUNC but it does not check for a function but for a
+#   define to exist. Consider a usage like:
+#
+#    AC_CHECK_DEFINE(__STRICT_ANSI__, CFLAGS="$CFLAGS -D_XOPEN_SOURCE=500")
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#
+#   This program is free software; you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation; either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 8
+
+AU_ALIAS([AC_CHECK_DEFINED], [AC_CHECK_DEFINE])
+AC_DEFUN([AC_CHECK_DEFINE],[
+AS_VAR_PUSHDEF([ac_var],[ac_cv_defined_$1])dnl
+AC_CACHE_CHECK([for $1 defined], ac_var,
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[
+  #ifdef $1
+  int ok;
+  #else
+  choke me
+  #endif
+]])],[AS_VAR_SET(ac_var, yes)],[AS_VAR_SET(ac_var, no)]))
+AS_IF([test AS_VAR_GET(ac_var) != "no"], [$2], [$3])dnl
+AS_VAR_POPDEF([ac_var])dnl
+])
+
+AU_ALIAS([AX_CHECK_DEFINED], [AX_CHECK_DEFINE])
+AC_DEFUN([AX_CHECK_DEFINE],[
+AS_VAR_PUSHDEF([ac_var],[ac_cv_defined_$2_$1])dnl
+AC_CACHE_CHECK([for $2 defined in $1], ac_var,
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <$1>]], [[
+  #ifdef $2
+  int ok;
+  #else
+  choke me
+  #endif
+]])],[AS_VAR_SET(ac_var, yes)],[AS_VAR_SET(ac_var, no)]))
+AS_IF([test AS_VAR_GET(ac_var) != "no"], [$3], [$4])dnl
+AS_VAR_POPDEF([ac_var])dnl
+])
+
+AC_DEFUN([AX_CHECK_FUNC],
+[AS_VAR_PUSHDEF([ac_var], [ac_cv_func_$2])dnl
+AC_CACHE_CHECK([for $2], ac_var,
+dnl AC_LANG_FUNC_LINK_TRY
+[AC_LINK_IFELSE([AC_LANG_PROGRAM([$1
+                #undef $2
+                char $2 ();],[
+                char (*f) () = $2;
+                return f != $2; ])],
+                [AS_VAR_SET(ac_var, yes)],
+                [AS_VAR_SET(ac_var, no)])])
+AS_IF([test AS_VAR_GET(ac_var) = yes], [$3], [$4])dnl
+AS_VAR_POPDEF([ac_var])dnl
+])# AC_CHECK_FUNC