Yes, you're right! I've used the MinGW package that can compile x86-64 and x86-32 binaries using just the -m64 and -m32 compiler flags. But, by default, the compiler produce x86-64 binaries since I've downloaded the x64 compiler version from this URL.
So, anyway, why not to make some x64 related fixes to let compile U++ in x86-64 mode just smooth and flawless?