Use simplified void_t for all compilers except GCC 4.x

[alexezeder]

Fixes #2149.

This PR moves a special void_t workaround for GCC 4.x:

fmt/include/fmt/core.h

Lines 1382 to 1385 in 640acba

	// A workaround for gcc 4.8 to make void_t work in a SFINAE context.
	template <typename... Ts> struct void_t_impl { using type = void; };
	template <typename... Ts>
	using void_t = typename detail::void_t_impl<Ts...>::type;

under the GCC < 5.0 macro check, adding a straightforward void_t implementation:

template <typename...> using void_t = void;

for other compilers.

This solves the problem with GCC 8.x that fails with the current implementation of void_t.

[vitaut]

Thank you!