OpenBSD patch at:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/mail/postfix/snapshot/patches/patch-src_tls_tls_h
$OpenBSD: patch-src_tls_tls_h,v 1.2 2016/02/25 09:39:49 sthen Exp $

Fix building with LibreSSL

--- src/tls/tls.h.orig	Sat Feb  6 15:09:41 2016
+++ src/tls/tls.h	Sun Feb 14 19:54:38 2016
@@ -89,7 +89,7 @@ extern const char *str_tls_level(int);
 #endif
 
  /* Backwards compatibility with OpenSSL < 1.1.0 */
-#if OPENSSL_VERSION_NUMBER < 0x10100000L
+#if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
 #define OpenSSL_version_num SSLeay
 #define OpenSSL_version SSLeay_version
 #define OPENSSL_VERSION SSLEAY_VERSION