BUILD: SSL: fine guard for SSL_CTX_add_server_custom_ext call

SSL_CTX_add_server_custom_ext is openssl specific function present
since openssl-1.0.2, let us define readable guard for it, not depending
on HA_OPENSSL_VERSION
diff --git a/include/haproxy/openssl-compat.h b/include/haproxy/openssl-compat.h
index 42349d7..564d1ea 100644
--- a/include/haproxy/openssl-compat.h
+++ b/include/haproxy/openssl-compat.h
@@ -45,6 +45,10 @@
 #define HAVE_SSL_CTX_SET_CIPHERSUITES
 #endif
 
+#if ((OPENSSL_VERSION_NUMBER >= 0x1000200fL) && !defined(OPENSSL_NO_TLSEXT) && !defined(LIBRESSL_VERSION_NUMBER) && !defined(OPENSSL_IS_BORINGSSL))
+#define HAVE_SL_CTX_ADD_SERVER_CUSTOM_EXT
+#endif
+
 #if (HA_OPENSSL_VERSION_NUMBER < 0x0090800fL)
 /* Functions present in OpenSSL 0.9.8, older not tested */
 static inline const unsigned char *SSL_SESSION_get_id(const SSL_SESSION *sess, unsigned int *sid_length)
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 732f586..9bb1d1c 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1495,7 +1495,7 @@
 #endif
 
 
-#if (HA_OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL)
+#ifdef HAVE_SL_CTX_ADD_SERVER_CUSTOM_EXT
 
 #define CT_EXTENSION_TYPE 18
 
@@ -3168,7 +3168,7 @@
 	}
 #endif
 
-#if (HA_OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL)
+#ifdef HAVE_SL_CTX_ADD_SERVER_CUSTOM_EXT
 	if (sctl_ex_index >= 0 && ckch->sctl) {
 		if (ssl_sock_load_sctl(ctx, ckch->sctl) < 0) {
 			memprintf(err, "%s '%s.sctl' is present but cannot be read or parsed'.\n",