This ensures that we test format string for the actual impl that is
being used. Depending if UCRT is used and if __USE_MINGW_ANSI_STDIO is
defined the implementation capabilities may differ.
Note that we don't explicitly define __USE_MINGW_ANSI_STDIO anymore,
this is defined implicitly in mingw headers, if needed. We don't support
ancient mingw versions that does not define it correctly for c11.