fixes Bug#28207237

disabled unused features and files in imported libevent 2.1.8 code

* disabled building samples and tests
* disabled running unit-tests and regression suite
* disable INSTALL
* allow all cmake build-types
* Removed evutil_secure_rng_add_bytes() from libevent API
  * it relies on arc4random_addrandom() which is either deprecated or not
    available on the target platforms
  * it is not used in our code
  * it is discouraged to be used in general

adaptations

* look for libevent 2.x headers in include/ instead of topsrcdir
* check for clock_gettime() in LIBRT only
* use proper socket-type on windows in xplugin
* unified LIBEVENT_ macro names for bundled and system
  * treat LIBEVENT_INCLUDE_DIRS is a system header
* require at least libevent-2.0