The `include/mbedtls/check_config.h` header file has a generic check
which requires `MBEDTLS_PLATFORM_C` to be defined on Windows when
building with MinGW and ancient MSVC versions.
The few mbedTLS files we include in builds without the module don't
really require anything from the mbedTLS platform file beside the
`mbedtls_platform_zeroize` function which we already override with a
custom one by defining `MBEDTLS_PLATFORM_ZEROIZE_ALT`.
So we simply add the same check and force the define in such case.
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