add --cores flag to multicore makefile

[xgrau]

Hi Darrin,

I have noticed that, on multicore systems, the default Makefile doesn't have a --cores flag, which causes compilation to fail. This small change fixed it for me. I guess there's better ways to do this, e.g. to autodetect the number of cores, but I'm not very familiar with snakemake and the such.

Cheers,

Xavi