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 "\".