Running Windows 10 at revision 34e3c17 and try to build a simple game produces quite a few fatal complaints about the -m64 flag and the error "sorry, unimplemented: 64-bit mode not compiled in"
Upon further investigation, this flag appears thanks to Compilers/Windows/gcc64.ey
even though, allegedly (or at least according to greg__), the compiler included is not 64-bit.
At any rate, one workaround is to always keep going into the Configuration Manager/Game Settings > ENIGMA > API > Compiler and change it from MinGW GCC G++ 64bit to just MinGW GCC G++
Alternatively, I've been told I can just delete gcc64.ey
Personally, I've taken the wimpy route of resetting to a prior stable revision due to other unrelated errors being present.