fix #167, make option ncpus work in linux.

ncpus does not work if graphlab is configured with  `--no_openmp` option.
````
    void set_ncpus(size_t n)
      {
#ifndef __NO_OPENMP__
          ncpus = n;
          omp_set_num_threads(ncpus);
#else
          ncpus = n; // fix 
#endif
      }
````


This is a bug in graphlab.
This bug can be fixed by pull this request.