We know that event:external will be configured first (because of its
priority). Take advantage of that here in libevent2022 by having it
refuse to configure / politely fail if event:external succeeded.
Also print out some additional lines in configure output indicating
what is going on (i.e., event:external succeeded, so this component
will be skipped, or event:external failed, so this component will be
used).
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
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