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diff --git a/m4/unlink.m4 b/m4/unlink.m4
new file mode 100644
index 0000000..af40377
--- /dev/null
+++ b/m4/unlink.m4
@@ -0,0 +1,130 @@
+# unlink.m4 serial 11
+dnl Copyright (C) 2009-2016 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_FUNC_UNLINK],
+[
+  AC_REQUIRE([gl_UNISTD_H_DEFAULTS])
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  AC_CHECK_HEADERS_ONCE([unistd.h])
+
+  dnl Detect FreeBSD 7.2, AIX 7.1, Solaris 9 bug.
+  AC_CACHE_CHECK([whether unlink honors trailing slashes],
+    [gl_cv_func_unlink_honors_slashes],
+    [touch conftest.file
+     # Assume that if we have lstat, we can also check symlinks.
+     if test $ac_cv_func_lstat = yes; then
+       ln -s conftest.file conftest.lnk
+     fi
+     AC_RUN_IFELSE(
+       [AC_LANG_PROGRAM(
+         [[#if HAVE_UNISTD_H
+           # include <unistd.h>
+           #else /* on Windows with MSVC */
+           # include <io.h>
+           #endif
+           #include <errno.h>
+         ]],
+         [[int result = 0;
+           if (!unlink ("conftest.file/"))
+             result |= 1;
+           else if (errno != ENOTDIR)
+             result |= 2;
+#if HAVE_LSTAT
+           if (!unlink ("conftest.lnk/"))
+             result |= 4;
+           else if (errno != ENOTDIR)
+             result |= 8;
+#endif
+           return result;
+         ]])],
+      [gl_cv_func_unlink_honors_slashes=yes],
+      [gl_cv_func_unlink_honors_slashes=no],
+      [case "$host_os" in
+                 # Guess yes on glibc systems.
+         *-gnu*) gl_cv_func_unlink_honors_slashes="guessing yes" ;;
+                 # If we don't know, assume the worst.
+         *)      gl_cv_func_unlink_honors_slashes="guessing no" ;;
+       esac
+      ])
+     rm -f conftest.file conftest.lnk])
+  case "$gl_cv_func_unlink_honors_slashes" in
+    *no)
+      REPLACE_UNLINK=1
+      ;;
+  esac
+
+  dnl Detect Mac OS X 10.5.6 bug: On read-write HFS mounts, unlink("..") or
+  dnl unlink("../..") succeeds without doing anything.
+  AC_CACHE_CHECK([whether unlink of a parent directory fails as it should],
+    [gl_cv_func_unlink_parent_fails],
+    [case "$host_os" in
+       darwin*)
+         dnl Try to unlink a subdirectory of /tmp, because /tmp is usually on a
+         dnl HFS mount on Mac OS X. Use a subdirectory, owned by the current
+         dnl user, because otherwise unlink() may fail due to permissions
+         dnl reasons, and because when running as root we don't want to risk
+         dnl destroying the entire /tmp.
+         if {
+              # Use the mktemp program if available. If not available, hide the error
+              # message.
+              tmp=`(umask 077 && mktemp -d /tmp/gtXXXXXX) 2>/dev/null` &&
+              test -n "$tmp" && test -d "$tmp"
+            } ||
+            {
+              # Use a simple mkdir command. It is guaranteed to fail if the directory
+              # already exists.  $RANDOM is bash specific and expands to empty in shells
+              # other than bash, ksh and zsh.  Its use does not increase security;
+              # rather, it minimizes the probability of failure in a very cluttered /tmp
+              # directory.
+              tmp=/tmp/gt$$-$RANDOM
+              (umask 077 && mkdir "$tmp")
+            }; then
+           mkdir "$tmp/subdir"
+           GL_SUBDIR_FOR_UNLINK="$tmp/subdir"
+           export GL_SUBDIR_FOR_UNLINK
+           AC_RUN_IFELSE(
+             [AC_LANG_SOURCE([[
+                #include <stdlib.h>
+                #if HAVE_UNISTD_H
+                # include <unistd.h>
+                #else /* on Windows with MSVC */
+                # include <direct.h>
+                # include <io.h>
+                #endif
+                int main ()
+                {
+                  int result = 0;
+                  if (chdir (getenv ("GL_SUBDIR_FOR_UNLINK")) != 0)
+                    result |= 1;
+                  else if (unlink ("..") == 0)
+                    result |= 2;
+                  return result;
+                }
+              ]])],
+             [gl_cv_func_unlink_parent_fails=yes],
+             [gl_cv_func_unlink_parent_fails=no],
+             [# If we don't know, assume the worst.
+              gl_cv_func_unlink_parent_fails="guessing no"
+             ])
+           unset GL_SUBDIR_FOR_UNLINK
+           rm -rf "$tmp"
+         else
+           gl_cv_func_unlink_parent_fails="guessing no"
+         fi
+         ;;
+       *)
+         gl_cv_func_unlink_parent_fails="guessing yes"
+         ;;
+     esac
+    ])
+  case "$gl_cv_func_unlink_parent_fails" in
+    *no)
+      REPLACE_UNLINK=1
+      AC_DEFINE([UNLINK_PARENT_BUG], [1],
+        [Define to 1 if unlink() on a parent directory may succeed])
+      ;;
+  esac
+])