Fix regression introduced in https://github.com/gtorrent/gtorrent-gtk/commit/8fc8ceac3e8d341f6c6fe9f578a9179099cd9522 resulting in logical error in Windows settings in Cmake and clarified reasons for flags in comments. -mwindows is a flag for NON debug builds. The only Windows specific CXXFLAG we could ever have is -mwindows (compiler specific flags for say VS would be a different variable), and even if we had another it should be added after initialization of the variable so comment is irrelevant.