Merge branch 'fix-struct-linger'

@jbech
  "Accidentally disabled by 0dda56a due to confusion between struct
  linger vs. SO_LINGER and #define vs. AC_DEFINE. Try adding synthetic
  #error test to confirm."

* fix-struct-linger:
  cmake: check for 'struct linger' existence
  test/bench*: prefix event-config.h macros after 0dda56a48e94
  test/bench_httpclient: restore SO_LINGER usage after 0dda56a48e94

Fixes: #444 (original pull-request)