build: manually update bootstrap from gnulib, and adapt

Updating to the latest bootstrap from gnulib involves more of a
change than usual, and updating to the latest gnulib would involve
its own set of challenges with the upcoming quoting changes, so
we update bootstrap manually and separately.
* bootstrap: Update from gnulib.
* Makefile.am: Initialize more variables to empty, so that gnulib.mk
can append to them with "+=".
* bootstrap.conf (gnulib_mk_hook): Remove.  No longer honored.
(bootstrap_post_import_hook): Instead, run the same command,
etc/prefix-gnulib-mk lib/$gnulib_mk, via slightly different API.
Temporarily disable "bootstrap_sync=true".
* etc/prefix-gnulib-mk: Don't prepend "lib/" to tokens like -I$(...
or "\".

bootstrap.conf

@@ -60,10 +60,12 @@ excluded_files='
     m4/visibility.m4
 '
 
-# Massage lib/gnulib.mk before using it later in the bootstrapping process.
-gnulib_mk_hook()
+gnulib_tool_option_extras='--symlink --makefile-name=gnulib.mk'
+
+bootstrap_post_import_hook()
 {
-  etc/prefix-gnulib-mk $1
+  # Massage lib/gnulib.mk before using it later in the bootstrapping process.
+  etc/prefix-gnulib-mk lib/$gnulib_mk
 }
 
 bootstrap_epilogue()
@@ -78,4 +80,4 @@ bootstrap_epilogue()
 # the bootstrap entry from the file .x-update-copyright, and add any
 # needed copyright years to the copyright statement in the bootstrap
 # script.
-bootstrap_sync=true
+# FIXME: temporary bootstrap_sync=true