Fix CUDA on Mac OS X. Fix CUDA in conjunction with mpifake.

nvcc on OS X compiles 32-bit binaries even on 64-bit systems. So check whether we're compiling for 64-bit Macs and set ```-m64``` accordingly.

Use ```-I./mpifake``` instead of ```-I/usr/include/mpi``` in nvcc flags if compiling with mpifake.