* Rely on 'event_core' vs 'event' to leave out protocol code.
  * Check for event2/thread.h
  * Add support for alternate platform includes
  * Send a prominent notice if libevent2 is not detected along with the
    consequences.
  * Remove event2/util.h from ntpdig/main.c as it is not used.
  * Disable ntpdig if libevent2 is not detected.
  * Build a small test binary with one func from each library.

This fixes #16