Inspect and control the number of threads when BLAS is implemented by Apple Accelerate

[ogrisel]

Follow-up on #135.

I am not sure if it's possible or not.

[ogrisel]

I have not conducted an extensive evaluation yet, but it seems that we do not suffer from oversubscription problems when calling vecLib's GEMM under OpenMP threads (for instance, in scikit-learn's KMeans). So maybe there is some kind of automated mechanism in Grand Central Dispatch that prevents the usual oversubscription problem we observed with other threaded BLAS libraries.

[ogrisel]

At least we could detect that Accelerate is linked, even if we cannot inspect or set the number of threads.

[ogrisel]

Apparently it's possible to tell vecLib to not use all threads via an environment variable: VECLIB_MAXIMUM_THREADS.

EDIT: it does not seem to have much effect on numpy workloads (matmul & SVD) linked against Accelerate on a Mac M1 host.