UltraStar-Deluxe/mxe#2 requires some updates to the projectM cwrapper to be able to build 64-bit compatible dlls for windows:

The issue was that the wrapper did not define its own ABI explicitly: it relied on an external DLLEXPORT macro for symbol export, and its function definitions relied on the compiler’s default calling convention instead of stating cdecl explicitly. On Windows, where calling convention is part of the ABI, that can lead to compatibility problems for Pascal callers.

The change was to define the wrapper ABI locally via PROJECTM_CWRAPPER_API and PROJECTM_CWRAPPER_CALL, and apply those macros to all exported declarations and definitions so symbol export and calling convention are explicit and work on every platform.