When #if-ery has both branches, i.e. has content in its main text and in its #else text, it's clearer to use a positive test rather than a negative one.  Although we know how double negatives work, people are more prone to getting confused, the more layers of negation they have to unwrap to see what's going on.  So I encourage you to change this to a #ifdef and swap the content around to suit.
Also: what about 64-bit MinGW ?  Does it still define the same "32-bit" symbol ?