Fix for C++14

Author: zauguin

hdr/sqlite_modern_cpp/log.h

@@ -8,6 +8,12 @@
 
 namespace sqlite {
 	namespace detail {
+		template<class>
+		using void_t = void;
+		template<class T, class = void>
+		struct is_callable : std::false_type {};
+		template<class Functor, class ...Arguments>
+		struct is_callable<Functor(Arguments...), void_t<decltype(std::declval<Functor>()(std::declval<Arguments>()...))>> : std::true_type {};
 		template<class Functor, class ...Functors>
 		class FunctorOverload: public Functor, public FunctorOverload<Functors...> {
 			public:
@@ -43,23 +49,23 @@ namespace sqlite {
 		};
 	}
 	template<class Handler>
-	typename std::enable_if<!std::is_callable<Handler(const sqlite_exception&)>::value>::type
+	typename std::enable_if<!detail::is_callable<Handler(const sqlite_exception&)>::value>::type
 	error_log(Handler &&handler);
 	template<class Handler>
-	typename std::enable_if<std::is_callable<Handler(const sqlite_exception&)>::value>::type
+	typename std::enable_if<detail::is_callable<Handler(const sqlite_exception&)>::value>::type
 	error_log(Handler &&handler);
 	template<class ...Handler>
 	typename std::enable_if<sizeof...(Handler)>=2>::type
 	error_log(Handler &&...handler) {
 		return error_log(detail::FunctorOverload<detail::WrapIntoFunctor<typename std::decay<Handler>::type>...>(std::forward<Handler>(handler)...));
 	}
 	template<class Handler>
-	typename std::enable_if<!std::is_callable<Handler(const sqlite_exception&)>::value>::type
+	typename std::enable_if<!detail::is_callable<Handler(const sqlite_exception&)>::value>::type
 	error_log(Handler &&handler) {
 		return error_log(std::forward<Handler>(handler), [](const sqlite_exception&) {});
 	}
 	template<class Handler>
-	typename std::enable_if<std::is_callable<Handler(const sqlite_exception&)>::value>::type
+	typename std::enable_if<detail::is_callable<Handler(const sqlite_exception&)>::value>::type
 	error_log(Handler &&handler) {
 		auto ptr = new auto([handler = std::forward<Handler>(handler)](int error_code, const char *errstr) mutable {
 			  switch(error_code & 0xFF) {