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 85ad742d4e)

Conflicts:

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

lib/Makefile.am

@@ -16,6 +16,16 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+AM_CFLAGS =
+AM_CPPFLAGS =
+BUILT_SOURCES =
+CLEANFILES =
+EXTRA_DIST =
+MOSTLYCLEANDIRS =
+MOSTLYCLEANFILES =
+SUFFIXES =
+noinst_LIBRARIES =
+
 include gnulib.mk
 
 AM_CFLAGS += $(WARN_CFLAGS)