GPU + MKL BLAS support

[enricodetoma]

#68 fixed GPU support, but was lost in a later commit.
With this pull request I introduced it again.

I also introduced MKL BLAS support with the flag WITH_MKL

[NeroBurner]

Thanks for the PR. Could you please split it into two PRs, one for MKL BLAS and one for the GULPU Support. I approve the MKL part right away, but I'm not knowledgeable enough with cuda to merge the GPU part.

Would those cuda changes be part of a newer updated upstream suitesparse release? If so I'd like to update to that instead of carrying patches to upstream code

[enricodetoma]

I can split if you want, but I just tell you that, for CUDA, I simply cherry picked the only commit of #68 (097d746) which @jlblancoc already merged in 2018, but was probably lost when SuiteSparse was updated. I myself I'm totally not knowledgeable enough about CUDA.

Then I compiled both SuiteSparse and Ceres solver with CUDA and run all tests in Ceres solver (some of which depend on SuiteSparse) and they all succeeded.

I agree that this patch should be also part of an upstream SuiteSparse, to avoid "being lost" again when SuiteSparse is updated.