cmake: cleanup - Remove a redundant check on netdb.h (EVENT__HAVE_NETDB_H). - Properly setup EVENT__SIZEOF_SSIZE_T for the case where "ssize_t" is not "int". - Remove unused EVENT__HAVE_PTHREAD. - Set EVENT__HAVE_LIBZ instead of EVENT__HAVE_ZLIB (and remove the latter). Closes: #391 (cherry-picked from PR)