[tor-bugs] #19758 [Core Tor/Tor]: Fix OpenBSD build bot errors

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#19758: Fix OpenBSD build bot errors
--------------------------+------------------------------
 Reporter:  teor          |          Owner:
     Type:  defect        |         Status:  new
 Priority:  Medium        |      Milestone:  Tor: 0.2.???
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:                |  Actual Points:
Parent ID:                |         Points:  0.5
 Reviewer:                |        Sponsor:
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Comment (by rubiate):

 The libevent2 problem is that commit
 265e40b481d23ea0ee8b4e1705fb013532d1f6b2 removed a check for the headers,
 and there's a conditional that relies on the result of that check, so that
 conditional always fails and configure falls back to linking libevent2
 with -levent, it just happens that this works on every platform that isn't
 OpenBSD (the whole conditional path that gets skipped was added
 specifically for OpenBSD in the first place).

 Since libevent2 is mandatory now I think it makes sense to add the header
 check back in and change the fall back path to an error.

 This patch gets tor successfully building HEAD (5e571900b341b) on OpenBSD
 -current amd64

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19758#comment:1>
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