Complete changeover from $bwin32/$cygwin to $ac_cv_sys_{win32,cygwin}.
Last batch of things to pull out of configure.ac.

* libevent_openssl.m4 renamed libevent_lib_checks.m4.
* Zlib test moved to libevent_lib_checks.m4.
* OpenSSL test overhauled to avoid action-at-a-distance dependencies on
  logic in configure.ac.