build: update gnulib submodule to latest

* bootstrap: Copy from latest Gnulib,
to fix --bootstrap-sync with other options.

bootstrap

@@ -3,7 +3,7 @@
 
 # Bootstrap this package from checked-out sources.
 
-scriptversion=2023-07-01.18; # UTC
+scriptversion=2023-12-10.18; # UTC
 
 # Copyright (C) 2003-2023 Free Software Foundation, Inc.
 #
@@ -37,7 +37,7 @@ medir=`dirname "$me"`
 
 # A library of shell functions for autopull.sh, autogen.sh, and bootstrap.
 
-scriptlibversion=2023-07-01.17; # UTC
+scriptlibversion=2023-12-10.18; # UTC
 
 # Copyright (C) 2003-2023 Free Software Foundation, Inc.
 #
@@ -659,7 +659,9 @@ Optional environment variables:
                            Use this if you already have gnulib sources
                            and history on your machine, and do not want
                            to waste your bandwidth downloading them again.
-  GNULIB_URL               Cloneable URL of the gnulib repository.
+  GNULIB_URL               URL of the gnulib repository.  The default is
+                           $default_gnulib_url,
+                           which is Gnulib's upstream repository.
 
 Options:
   --bootstrap-sync         if this bootstrap script is not identical to
@@ -775,7 +777,7 @@ autopull()
   if $use_gnulib || $bootstrap_sync; then
     prepare_GNULIB_SRCDIR
     if $bootstrap_sync; then
-      upgrade_bootstrap
+      upgrade_bootstrap "$@"
     fi
   fi
 
@@ -1347,7 +1349,9 @@ Optional environment variables:
                            have gnulib sources on your machine, and
                            do not want to waste your bandwidth downloading
                            them again.
-  GNULIB_URL               Cloneable URL of the gnulib repository.
+  GNULIB_URL               URL of the gnulib repository.  The default is
+                           $default_gnulib_url,
+                           which is Gnulib's upstream repository.
 
 Options:
 
@@ -1492,7 +1496,7 @@ check_build_prerequisites $use_git
 
 if $bootstrap_sync; then
   prepare_GNULIB_SRCDIR
-  upgrade_bootstrap
+  upgrade_bootstrap "$@"
   # Since we have now upgraded if needed, no need to try it a second time below.
   bootstrap_sync=false
 fi