- Always use the external component when configure'd with --with-libevent=external
- Fix the external libevent library version detection
by testing _EVENT_NUMERIC_VERSION and EVENT__NUMERIC_VERSION macros
- Use the event2/event.h header (event.h is deprecated since libevent 2.0
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
(cherry picked from commit 35e77a286c369c1be66ee7ef9ad5ec2faef47edb)