BLD: introduce use of BLAS_LIBS and LAPACK_LIBS in distutils/system_info (#18737)

This enables the behaviour of the build honouring the environment
variables NPY_BLAS_LIBS, NPY_CBLAS_LIBS, and NPY_LAPACK_LIBS
to select any standard-conforming implementation of the f77
interfaces.

This is needed to sanely have automated builds provide BLAS libs
in a way common to multiple packages (using some environment variable,
BLAS_LIBS being canonical, but variants exist). Hacking a site.cfg
is fine for a single user, but does not scale well. As example,
pkgsrc, the NetBSD packages collection, offers differing builds
of Netlib or OpenBLAS, the latter in variables openblas,
openblas_openmp, and openblas_pthread. With this patch, differing
builds can just use different variable values like

NPY_BLAS_LIBS=-lblas NPY_CBLAS_LIBS=-lcblas \
  NPY_LAPACK_LIBS='-lapack -lblas' \
  python setup.py build

NPY_BLAS_LIBS=-lopenblas_pthread NPY_CBLAS_LIBS= \
  NPY_LAPACK_LIBS='-lopenblas_pthread' \
  python setup.py build

(NPY_LAPACK_LIBS contains reference to BLAS, too, for packages that only
use LAPACK and would miss underlying BLAS otherwise for static linking.)

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