define WIN32 on windows builds

[nutzboi]

Improves (not sure / haven't tested if it replaces) what 6c23f94 achieves. It had broken in the first place because the macro WIN32 was deprecated and replaced only with _WIN32 on most compilers. This commit explicitly defines WIN32 in CMakeLists (along with the kimden-defined WIN_BUILD) when compiling with MSVC or MINGW.

This fixes music being broken in the game for me. 9d8cecc can also now be reverted lol.

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