Cmake can't properly determine 64bit host

luajit build fails because of pointer size
mismatch, because the OSX host isn't properly
describing itself as 64bit via uname -p
(returns i386?!)

script hack in cmake/third-party/luajit/CMakeLists.txt
so luajit cross-builds 32bit

taken from https://github.com/moai/moai-dev/issues/865