Provide default configuration for OpenMP, MKL, Cuda, -flto
The build options are becoming quite complicated and there is no way to set certain options directly in the nim files.

Cuda:
https://github.com/mratsim/Arraymancer/blob/ab2b88debd4cc845bae6e19582370ba9373bc952/arraymancer.nimble#L35-L47

MKL and MKL + OpenMP
https://github.com/mratsim/Arraymancer/blob/ab2b88debd4cc845bae6e19582370ba9373bc952/arraymancer.nimble#L110-L128

Arraymancer should ship with arraymancer.cuda.nim.cfg, arraymancer.mkl_openmp.nim.cfg so setup is easier.

