Check that Libevent header version matches Libevent library version.
Unfortunately, old Libevents don't _put_ a version in their headers, so this can get a little tricky. Fortunately, the only binary-compatibility issue we care about is the size of struct event. Even more fortunately, Libevent 2.0 will let us keep binary compatiblity forever by letting us decouple ourselves from the structs, if we like. svn:r18014