Fix locale-related errors for MinGW

Hadn't compiled on MinGW lately; tried to this morning and it failed because of locale functionality introduced in the OCCT 6.6.0 changes. Basically from my understanding, MSVCRT used by MinGW has very limited functionality for locale-specific functions that are supported by modern MSVC (e.g. vsprintf_l). The _WIN32 define falsely triggers the use of these unsupported functions and so compilation fails.