BUILD: check for libressl to be able to build against it

[wt: might be worth backporting it to 1.6]
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 3f66af1..c67cd56 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -714,7 +714,7 @@
 
 #endif
 
-#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL)
+#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL && !defined LIBRESSL_VERSION_NUMBER)
 
 #define CT_EXTENSION_TYPE 18
 
@@ -1736,7 +1736,7 @@
 	}
 #endif
 
-#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL)
+#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL && !defined LIBRESSL_VERSION_NUMBER)
 	if (sctl_ex_index >= 0) {
 		ret = ssl_sock_load_sctl(ctx, path);
 		if (ret < 0) {
@@ -5348,7 +5348,7 @@
 	.init     = ssl_sock_init,
 };
 
-#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL)
+#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL && !defined LIBRESSL_VERSION_NUMBER)
 
 static void ssl_sock_sctl_free_func(void *parent, void *ptr, CRYPTO_EX_DATA *ad, int idx, long argl, void *argp)
 {
@@ -5381,7 +5381,7 @@
 	SSL_library_init();
 	cm = SSL_COMP_get_compression_methods();
 	sk_SSL_COMP_zero(cm);
-#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL)
+#if (OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined OPENSSL_NO_TLSEXT && !defined OPENSSL_IS_BORINGSSL && !defined LIBRESSL_VERSION_NUMBER)
 	sctl_ex_index = SSL_CTX_get_ex_new_index(0, NULL, NULL, NULL, ssl_sock_sctl_free_func);
 #endif
 	sample_register_fetches(&sample_fetch_keywords);