For MinGW.
As per issue 634, Stua "will be deleted from stb.h" soonish. That was 3 years ago, which should be plenty of warning, and the language has been de-facto orphaned and undocumented for a good while longer than that. Fixes issue #634.
It was never designed to be used by anyone but Sean and has numerous problems; new code should definitely not be using this.