maint: update gnulib and fix build failures

I got failures when trying to bootstrap:

bootstrap: getting translations into po/.reference for m4...
receiving incremental file list
rsync: change_dir "/latest/m4" (in tp) failed: No such file or directory (2)

since m4-1.4 has no translation files, and the translation project
dropped the stale .po files for the unreleased 1.9 development branch.

Once that was fixed, I also got compilation failures, from an
incomplete update to the gnulib execute module:

builtin.c: In function 'm4_syscmd':
builtin.c:968:44: error: passing argument 3 of 'execute' from incompatible pointer type [-Werror=incompatible-pointer-types]
  968 |   status = execute (ARG (0), SYSCMD_SHELL, prog_args, NULL, false,
      |                                            ^~~~~~~~~
      |                                            |
      |                                            const char **

Fixes: 4e5c2c0157

* gnulib: Update to latest.
* bootstrap.conf (copyright_holder): Silence bootstrap warning.
(m4_bootstrap_options_prep): Turn off po update.

3 files changed, 14 insertions, 2 deletions

diff --git a/bootstrap.conf b/bootstrap.conf
index 69a4f150..6b608f25 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -1,4 +1,4 @@
-# bootstrap.conf (GNU M4) version 2010-09-10
+# bootstrap.conf (GNU M4) version 2021-04-01
 # Written by Gary V. Vaughan, 2010
 
 # Copyright (C) 2010, 2014, 2016-2017, 2020 Free Software Foundation,
@@ -32,6 +32,9 @@ buildreq='
         xz         -           http://tukaani.org/xz
 '
 
+# Owner of the copyright of this package.
+copyright_holder='Free Software Foundation, Inc.'
+
 # Instructions on how to install packages in $buildreq.
 buildreq_readme=HACKING
 
@@ -80,6 +83,15 @@ m4_install_texinfo_diff_driver ()
 }
 func_add_hook func_prep m4_install_texinfo_diff_driver
 
+# m4_bootstrap_options_prep
+# -------------------------
+# Skip po import until we integrate translation.
+m4_bootstrap_options_prep ()
+{
+    opt_skip_po=:
+}
+func_add_hook func_options_prep m4_bootstrap_options_prep
+
 
 # Local variables:
 # mode: shell-script
diff --git a/gnulib b/gnulib
-Subproject 4cfff6810c595e60cf4411dc9bac83c0328e881
+Subproject 152be6db1b016ed2a510ddc3895a937b21fbd53
diff --git a/m4/gnulib-cache.m4 b/m4/gnulib-cache.m4
index 8367ac15..13c88156 100644
--- a/m4/gnulib-cache.m4
+++ b/m4/gnulib-cache.m4
@@ -1,4 +1,4 @@
-# Copyright (C) 2002-2020 Free Software Foundation, Inc.
+# Copyright (C) 2002-2021 Free Software Foundation, Inc.
 #
 # This file is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by