Patch to quite another Mingw Build warning (mmsystem.h)

[Eric Pouech]

Steven Edwards a écrit :

> +#ifndef __MINGW__

using #ifndef SEEK_SET would much better
it's most of the time a bad idea to use #ifdef MYOS to check
for the presence/absence of a feature
the best way is to check for the feature itself, and not
infer it from the OS...