To build a 64-bit binary on windows you need the 64-bit version of the mingw compiler. Mingw-W64 is one possible distribution you can use. You can find a list of downloads here.
Additionally, you can also find Dev-C++ setup bundled with mingw 64-bit compiler under Orwell Dev-C++'s download section. Make sure you choose "TDM-GCC x64 4.7.1" either setup or portable.