Provide CRYPTO_zalloc to fix build with LibreSSL

Index: src/SWIG/_lib11_compat.i
--- src/SWIG/_lib11_compat.i.orig
+++ src/SWIG/_lib11_compat.i
@@ -8,7 +8,7 @@
  */
 
 %{
-#if OPENSSL_VERSION_NUMBER < 0x10100000L
+#if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
 
 #include <string.h>
 #include <openssl/engine.h>
@@ -24,6 +24,8 @@ static void *CRYPTO_zalloc(size_t num, const char *fil
       return ret;
 }
 
+#endif
+#if OPENSSL_VERSION_NUMBER < 0x10100000L
 #include <openssl/bn.h>
 
 #ifndef BN_F_BN_GENCB_NEW