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

Starting this week, Gnulib supports generating a "non-recursive" Automake
Makefile.am snippet https://lists.gnu.org/r/bug-gnulib/2021-12/msg00085.html.

* bootstrap.conf: Use gnulib-tool's --automake-subdir.

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
 }

build-aux/.gitignore

@@ -19,7 +19,6 @@
 /libtool-reloc
 /mdate-sh
 /missing
-/prefix-gnulib-mk
 /reloc-ldflags
 /relocatable.sh.in
 /test-driver

m4/.gitignore

@@ -142,7 +142,6 @@
 /musl.m4
 /nls.m4
 /nocrash.m4
-/non-recursive-gnulib-prefix-hack.m4
 /obstack-printf.m4
 /obstack.m4
 /off_t.m4