Since we want the choice of blas/lapack to be configurable by .pc files we need to have numpy and scipy obey to pkg-config.

This is complicated by the fact that numpy/scipy's build system will look for

1. mkl
2. openblas
3. threaded atlas 3.10.x (tatlas.so)
4. single thread atlas 3.10.x (satlas.so)
5. threaded atlas (any versions)
6. single thread atlas (any versions)
7. accelerate on OS X
8. blas/lapack configuration provided by the user
9. blas/lapack source provided by the user

In that order from (1) to (9). Note that the user's wishes are ignored unless everything else fails. Getting numpy/scipy to obey the user wishes, whatever they are, means using not well documented variables to disable the auto-detections steps 1 to 6.

CC: @jdemeyer

Component: packages: standard

Author: François Bissey

Branch/Commit: af70376

Reviewer: Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/20157