use NULL for context with zero context length for modern clang
anyway, use_context is disabled (last param)
--- a/src/tlsopensslclient.cpp
+++ b/src/tlsopensslclient.cpp
@@ -51,7 +51,7 @@ namespace gloox
     {
       unsigned char buf[32];
       const char* const label = "EXPORTER-Channel-Binding";
-      SSL_export_keying_material( m_ssl, buf, 32, label, strlen( label ), { 0 }, 1, 0 );
+      SSL_export_keying_material( m_ssl, buf, 32, label, strlen( label ), NULL, 0, 0 );
       return std::string( reinterpret_cast<char* const>( buf ), 32 );
     }
     else