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diff --git a/m4/posixver.m4 b/m4/posixver.m4
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+# posixver.m4 serial 12
+dnl Copyright (C) 2002-2006, 2009-2020 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_POSIXVER],
+[
+  AC_REQUIRE([gl_DEFAULT_POSIX2_VERSION])
+])
+
+# Set the default level of POSIX conformance at configure-time.
+# Build with './configure DEFAULT_POSIX2_VERSION=199209 ...' to
+# support the older version, thus preserving portability with
+# scripts that use sort +1, tail +32, etc.
+# Note however, that this breaks tools that might run commands
+# like 'sort +some-file' that conform with the newer standard.
+AC_DEFUN([gl_DEFAULT_POSIX2_VERSION],
+[
+  AC_MSG_CHECKING([for desired default level of POSIX conformance])
+  gl_default_posix2_version=none-specified
+  if test -n "$ac_cv_env_DEFAULT_POSIX2_VERSION_set"; then
+    gl_default_posix2_version=$ac_cv_env_DEFAULT_POSIX2_VERSION_value
+    AC_DEFINE_UNQUOTED([DEFAULT_POSIX2_VERSION],
+      $gl_default_posix2_version,
+      [Define the default level of POSIX conformance. The value is of
+       the form YYYYMM, specifying the year and month the standard was
+       adopted. If not defined here, it defaults to the value of
+       _POSIX2_VERSION in <unistd.h>. Define to 199209 to default to
+       POSIX 1003.2-1992, which makes standard programs like 'head',
+       'tail', and 'sort' accept obsolete options like '+10' and
+       '-10'. Define to 200112 to default to POSIX 1003.1-2001, which
+       makes these standard programs treat leading-'+' operands as
+       file names and require modern usages like '-n 10' instead of
+       '-10'. Whether defined here or not, the default can be
+       overridden at run time via the _POSIX2_VERSION environment
+       variable.])
+  fi
+  AC_MSG_RESULT([$gl_default_posix2_version])
+  AC_ARG_VAR(
+    [DEFAULT_POSIX2_VERSION],
+    [POSIX version to default to; see 'config.hin'.])
+])