Add workaround for libstdc++ < 5.0

[gnzlbg]

libstdc++ < 5.0 doesn't provide std::is_trivially_copyable but the old proposed type traits. The following workaround can be added to support libstdc++ < 5.0

#if defined(__GLIBCXX__) && __GLIBCXX__ < 20150801
namespace std {
template <typename T>
struct is_trivially_copyable : integral_constant<bool, __has_trivial_copy(T)> {
};
}  // namespace std
#endif  // GLIBCXX macro

[mpark]

Closing this as is_trivially_copyable is no longer used.