BUILD: enable several LibreSSL hacks, including

SSL_SESSION_get0_id_context is introduced in LibreSSL-2.7.0
async operations are not supported by LibreSSL
early data is not supported by LibreSSL
packet_length is removed from SSL struct in LibreSSL
diff --git a/src/cli.c b/src/cli.c
index 88fbae3..e91e33b 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1002,7 +1002,7 @@
 			     (fdt.iocb == poller_pipe_io_handler) ? "poller_pipe_io_handler" :
 			     (fdt.iocb == mworker_accept_wrapper) ? "mworker_accept_wrapper" :
 #ifdef USE_OPENSSL
-#if (OPENSSL_VERSION_NUMBER >= 0x1010000fL) && !defined(OPENSSL_NO_ASYNC)
+#if (OPENSSL_VERSION_NUMBER >= 0x1010000fL) && !defined(OPENSSL_NO_ASYNC) && !defined(LIBRESSL_VERSION_NUMBER)
 			     (fdt.iocb == ssl_async_fd_free) ? "ssl_async_fd_free" :
 			     (fdt.iocb == ssl_async_fd_handler) ? "ssl_async_fd_handler" :
 #endif