build: manually update bootstrap from gnulib, and adapt

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.
* lib/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.
(gnulib_tool_option_extras): Generate gnulib.mk.
(cherry picked from commit 85ad742d4e94345d5ac07755f6722344cfd49603)

Conflicts:

	Makefile.am
	bootstrap.conf
	etc/prefix-gnulib-mk