Fix build with Clang for Windows

[Clemoot]

I am trying to compile and use xsimd with Clang on Windows.

I just discovered that Clang for Windows doesn't define __GNUC__ but instead defines _MSC_VER. That makes the compiler choose a version of XSIMD_INLINE that it does not support.

The only solution I found that do supports both platforms is by extending #if defined(__GNUC__) with (defined(_MSC_VER) && defined(__clang__).

[serge-sans-paille]

Alternative: https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fattribute.html
See https://godbolt.org/z/YYjfGzqYM

[serge-sans-paille]

(and thanks for spotting this!)

[Clemoot]

I updated the changes, which in fact are way cleaner with your suggestion.