Another version of this patch (different function names):

--- src/openvpn/openssl_compat.h	2018-03-24 15:10:13.247855613 +0100
+++ src/openvpn/openssl_compat.h	2018-03-24 15:11:56.094324516 +0100
@@ -661,7 +661,7 @@
 #define RSA_F_RSA_OSSL_PRIVATE_ENCRYPT       RSA_F_RSA_EAY_PRIVATE_ENCRYPT
 #endif
 
-#ifndef SSL_CTX_get_min_proto_version
+#if 0
 /** Return the min SSL protocol version currently enabled in the context.
  *  If no valid version >= TLS1.0 is found, return 0. */
 static inline int
@@ -684,7 +684,7 @@
 }
 #endif /* SSL_CTX_get_min_proto_version */
 
-#ifndef SSL_CTX_get_max_proto_version
+#if 0
 /** Return the max SSL protocol version currently enabled in the context.
  *  If no valid version >= TLS1.0 is found, return 0. */
 static inline int
@@ -711,7 +711,7 @@
 }
 #endif /* SSL_CTX_get_max_proto_version */
 
-#ifndef SSL_CTX_set_min_proto_version
+#if 0
 /** Mimics SSL_CTX_set_min_proto_version for OpenSSL < 1.1 */
 static inline int
 SSL_CTX_set_min_proto_version(SSL_CTX *ctx, long tls_ver_min)
@@ -740,7 +740,7 @@
 }
 #endif /* SSL_CTX_set_min_proto_version */
 
-#ifndef SSL_CTX_set_max_proto_version
+#if 0
 /** Mimics SSL_CTX_set_max_proto_version for OpenSSL < 1.1 */
 static inline int
 SSL_CTX_set_max_proto_version(SSL_CTX *ctx, long tls_ver_max)

Tried to use this patch with my openvpn-build fork, but for some reason it still complains about missing symbols in the link phase when cross-compiling (for win64), and in that situation autoconf is not called (so I'm unable to use the patch which I used when building with LibreSSL 2.6.4).