In MSVC 2019, vsnprintf became a real function that correctly matches the C99 standard, and as it's no longer a macro, it cannot be #undef'd. This causes a Macro redefinition conflict with the Standard Library function.
I assume that the only two options here were Mingw and MSVC. I would guess Cygwin would also fall under Mingw. I also assumed that the "not cross-compiling" meant MSVC. So this condition should now correctly target either of those two options.

Compiled on MSVC, and it no longer errored.