make use of gnulib-tool's --automake-subdir option

[Bruno Haible]

Hi,

Starting this week, Gnulib supports generating a "non-recursive" Automake
Makefile.am snippet [1].

Here's a proposed patch to make use of it in Bison. Tested through a VPATH
build "../configure && make && make check".

We plan to deprecate the older approach.

Bruno

[1] https://lists.gnu.org/archive/html/bug-gnulib/2021-12/msg00085.html

diff --git a/bootstrap.conf b/bootstrap.conf
index 1d8614e7..6b818fc6 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -34,7 +34,6 @@ gnulib_modules='
   linked-list
   malloc-gnu
   mbfile mbswidth
-  non-recursive-gnulib-prefix-hack
   obstack
   obstack-printf
   perror progname
@@ -80,13 +79,10 @@ XGETTEXT_OPTIONS_RUNTIME=$XGETTEXT_OPTIONS'\\\
  --keyword=YY_ \\\
 '
 
-gnulib_tool_option_extras='--symlink --conditional-dependencies --makefile-name=gnulib.mk --po-base=gnulib-po --po-domain=bison'
+gnulib_tool_option_extras='--symlink --conditional-dependencies --makefile-name=gnulib.mk --automake-subdir --po-base=gnulib-po --po-domain=bison'
 
 bootstrap_post_import_hook()
 {
-  # Massage lib/gnulib.mk before using it later in the bootstrapping process.
-  build-aux/prefix-gnulib-mk --lib-name="$gnulib_name" "lib/$gnulib_mk"
-
   # Ensure that ChangeLog exists, for automake.
   test -f ChangeLog || touch ChangeLog
 }