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diff --git a/m4/extensions.m4 b/m4/extensions.m4
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+++ b/m4/extensions.m4
@@ -0,0 +1,123 @@
+# serial 10  -*- Autoconf -*-
+# Enable extensions on systems that normally disable them.
+
+# Copyright (C) 2003, 2006-2011 Free Software Foundation, Inc.
+# This file is free software; the Free Software Foundation
+# gives unlimited permission to copy and/or distribute it,
+# with or without modifications, as long as this notice is preserved.
+
+# This definition of AC_USE_SYSTEM_EXTENSIONS is stolen from CVS
+# Autoconf.  Perhaps we can remove this once we can assume Autoconf
+# 2.62 or later everywhere, but since CVS Autoconf mutates rapidly
+# enough in this area it's likely we'll need to redefine
+# AC_USE_SYSTEM_EXTENSIONS for quite some time.
+
+# If autoconf reports a warning
+#     warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
+# or  warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
+# the fix is
+#   1) to ensure that AC_USE_SYSTEM_EXTENSIONS is never directly invoked
+#      but always AC_REQUIREd,
+#   2) to ensure that for each occurrence of
+#        AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+#      or
+#        AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+#      the corresponding gnulib module description has 'extensions' among
+#      its dependencies. This will ensure that the gl_USE_SYSTEM_EXTENSIONS
+#      invocation occurs in gl_EARLY, not in gl_INIT.
+
+# AC_USE_SYSTEM_EXTENSIONS
+# ------------------------
+# Enable extensions on systems that normally disable them,
+# typically due to standards-conformance issues.
+# Remember that #undef in AH_VERBATIM gets replaced with #define by
+# AC_DEFINE.  The goal here is to define all known feature-enabling
+# macros, then, if reports of conflicts are made, disable macros that
+# cause problems on some platforms (such as __EXTENSIONS__).
+AC_DEFUN_ONCE([AC_USE_SYSTEM_EXTENSIONS],
+[AC_BEFORE([$0], [AC_COMPILE_IFELSE])dnl
+AC_BEFORE([$0], [AC_RUN_IFELSE])dnl
+
+  AC_REQUIRE([AC_CANONICAL_HOST])
+
+  AC_CHECK_HEADER([minix/config.h], [MINIX=yes], [MINIX=])
+  if test "$MINIX" = yes; then
+    AC_DEFINE([_POSIX_SOURCE], [1],
+      [Define to 1 if you need to in order for `stat' and other
+       things to work.])
+    AC_DEFINE([_POSIX_1_SOURCE], [2],
+      [Define to 2 if the system does not provide POSIX.1 features
+       except with this defined.])
+    AC_DEFINE([_MINIX], [1],
+      [Define to 1 if on MINIX.])
+  fi
+
+  dnl HP-UX 11.11 defines mbstate_t only if _XOPEN_SOURCE is defined to 500,
+  dnl regardless of whether the flags -Ae or _D_HPUX_SOURCE=1 are already
+  dnl provided.
+  case "$host_os" in
+    hpux*)
+      AC_DEFINE([_XOPEN_SOURCE], [500],
+        [Define to 500 only on HP-UX.])
+      ;;
+  esac
+
+  AH_VERBATIM([__EXTENSIONS__],
+[/* Enable extensions on AIX 3, Interix.  */
+#ifndef _ALL_SOURCE
+# undef _ALL_SOURCE
+#endif
+/* Enable general extensions on MacOS X.  */
+#ifndef _DARWIN_C_SOURCE
+# undef _DARWIN_C_SOURCE
+#endif
+/* Enable GNU extensions on systems that have them.  */
+#ifndef _GNU_SOURCE
+# undef _GNU_SOURCE
+#endif
+/* Enable threading extensions on Solaris.  */
+#ifndef _POSIX_PTHREAD_SEMANTICS
+# undef _POSIX_PTHREAD_SEMANTICS
+#endif
+/* Enable extensions on HP NonStop.  */
+#ifndef _TANDEM_SOURCE
+# undef _TANDEM_SOURCE
+#endif
+/* Enable general extensions on Solaris.  */
+#ifndef __EXTENSIONS__
+# undef __EXTENSIONS__
+#endif
+])
+  AC_CACHE_CHECK([whether it is safe to define __EXTENSIONS__],
+    [ac_cv_safe_to_define___extensions__],
+    [AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM([[
+#         define __EXTENSIONS__ 1
+          ]AC_INCLUDES_DEFAULT])],
+       [ac_cv_safe_to_define___extensions__=yes],
+       [ac_cv_safe_to_define___extensions__=no])])
+  test $ac_cv_safe_to_define___extensions__ = yes &&
+    AC_DEFINE([__EXTENSIONS__])
+  AC_DEFINE([_ALL_SOURCE])
+  AC_DEFINE([_DARWIN_C_SOURCE])
+  AC_DEFINE([_GNU_SOURCE])
+  AC_DEFINE([_POSIX_PTHREAD_SEMANTICS])
+  AC_DEFINE([_TANDEM_SOURCE])
+])# AC_USE_SYSTEM_EXTENSIONS
+
+# gl_USE_SYSTEM_EXTENSIONS
+# ------------------------
+# Enable extensions on systems that normally disable them,
+# typically due to standards-conformance issues.
+AC_DEFUN_ONCE([gl_USE_SYSTEM_EXTENSIONS],
+[
+  dnl Require this macro before AC_USE_SYSTEM_EXTENSIONS.
+  dnl gnulib does not need it. But if it gets required by third-party macros
+  dnl after AC_USE_SYSTEM_EXTENSIONS is required, autoconf 2.62..2.63 emit a
+  dnl warning: "AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS".
+  dnl Note: We can do this only for one of the macros AC_AIX, AC_GNU_SOURCE,
+  dnl AC_MINIX. If people still use AC_AIX or AC_MINIX, they are out of luck.
+  AC_REQUIRE([AC_GNU_SOURCE])
+
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+])