build: fix example build on MacOS X

build: fix example build on MacOS X

Partial reversion of commit 76d87a59, now that bootstrap is smarter.

* .gnulib: Update to latest, for poll and bootstrap fixes.
* bootstrap: Resync from gnulib.
* autogen.sh: Drop redundant tool checks; bootstrap does them
better, by honoring environment variables.
* examples/domain-events/events-c/Makefile.am (INCLUDES)
(event_test_LDADD): Use gnulib library during build.
* bootstrap.conf (gnulib_tool_option_extras): Revert --libtool
addition, now that updated bootstrap does it for us.
Reported by Justin Clift.