support gcc8 with defined MAGIC_ENUM_NO_CHECK_SUPPORT and with specialized magic_enum::customize::enum_name

[schaumb]

I tried to run examples and tests with gcc8, with manually specialized magic_enum::customize::enum_name.
I know that GCC8 is not supported, but these changes help us to make compatible our code with older GCC too only with the custom template function.

---

I replaced
#if defined(MAGIC_ENUM_SUPPORTED) && MAGIC_ENUM_SUPPORTED
macros to
if constexpr (supported<E>::value) {

at class name getter n(), and operator<<
These functions work with custom magic_enum::customize::enum_name, so it is enough to check supported flag.

---

I replaced detail::min_v<E> template parameter E, because E can be a reference, and std::underlying_type is undefined in some compilers when the template parameter is a reference.

---

I switched values_v and entries_v from auto to std::array, because GCC8 is not able to deduct the whole type, but std::array template parameters. This change should not change the newer compilers (and it is not a big change)

[Neargye]

detail::min_v with E
It's a good catch, there really should have been a D, it was a typo.

In general, I like the changes, I will look at them a little more before the merge.

[Neargye]

Corrected the style and made the same check for n<E, V>(). Can you check if these changes are ok for you? and I think this PR is ready

[schaumb]

These changes are ok! Thanks :)

[Neargye]

Thanks!

[JuliaGut]

@schaumb I really like the Magic Enum library, but unfortunately I only have a gcc8 compiler available for my project.
Do you know, if it is possible to use the function "magic_enum::enum_name" with gcc8 compiler? If so, could you tell me which version of the library to use for it?

[schaumb]

Hi @JuliaGut
So the Magic Enum lib is not supported on gcc-8, because the hack used in this lib is not available on gcc-8.

If you want to use it, you need to specify all member's names individually.
It can be done like this

enum class EN {
    asd,
    fgh
};

namespace magic_enum::customize {
template<>
constexpr customize_t enum_name<EN>(EN e) {
    switch(e) {
        case EN::asd: return "asd";
        case EN::fgh: return "fgh";
    }
    return invalid_tag;
}
}

[JuliaGut]

Thanks a lot for the quick response! 🥇