Core (Build): Get MSVC to use correct values for the __cplusplus define

[kaixiong]

Rant: As usual, Microsoft does its own thing.

References:

• Documentation on Microsoft Learn (link)
• Thread on CMake (link)

[hartwork]

@kaixiong PS: On a side note we're saying goodbye to Visual Studio <2017 (at least in part) here. Fine in my book, but important to know.

[kaixiong]

@hartwork I ported Libvisual to Win32 (writing the WaveIn input in the process) and have never tried to compile it on MSVC. Maybe I should try one of these days and see where the gaps are. Interestingly enough, MSVC is now ahead of both GCC and Clang in terms of the latest C++ standard support.

[hartwork]

@kaixiong we can make the CI cover MSVC at some point.

[kaixiong]

@kaixiong PS: On a side note we're saying goodbye to Visual Studio <2017 (at least in part) here. Fine in my book, but important to know.

@hartwork We already are, master requires C++20 to build.

[hartwork]

@kaixiong I guess you're right — https://en.cppreference.com/w/cpp/compiler_support#cpp20