workaround gcc 8 and below bug

gcc 8 and below aren't able to deduce type template parameters if it is
an empty variadic parameter list.

Author: lukevalenty

test/test_nexus.cpp

@@ -5,7 +5,7 @@
 using cib::detail::int_;
 
 struct EmptyConfig {
-    constexpr static auto config = cib::config();
+    constexpr static auto config = cib::config<>();
 };
 
 TEST_CASE("an empty configuration should compile and initialize") {