_Static_assert on gcc < 6

[adeason]

Commit a4194fd changed HEDLEY_STATIC_ASSERT to require gcc >= 6 to use _Static_assert, but it seems to work fine as far back as at least gcc 4.6.4, as far as I can tell. I'm just wondering if there is some issue with gcc pre-6 that I'm not immediately seeing? It works even with -std=c89. I do see a warning with -pedantic, but that also happens with gcc 6.

https://godbolt.org/z/6q5j856na

https://godbolt.org/z/nKaP41WeP