Use _WIN32 instead of _MSC_VER

[giordano]

This macro should check that the target is Windows, not that the compiler is Microsoft Visual Studio Code, which would prevent compilation with MinGW.

There are other instances of _MSC_VER which look suspiciously wrong, but this is the only one I found strictly necessary to compile libucl with MinGW.

[vstakhov]

Is _WIN32 defined on 64 bit windows platforms?

[giordano]

It is:

_WIN32 Defined as 1 when the compilation target is 32-bit ARM, 64-bit ARM, x86, or x64. Otherwise, undefined.

That's the only macro that should be used to test a Windows target. _MSC_VER is only reliably defined by Visual Studio Code, it may not be defined by other compilers targeting Windows.