exact-int.h `_STDC_VERSION___` instead of `__STDC_VERSION__`

[bowtoes]

Is a typo, the very first instance in if defined(...) is missing a single leading underscore.

portable-snippets/exact-int/exact-int.h

Line 45 in 84abba9

# if defined(_STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)