ENH: allowed external users to select BLAS and LAPACK library link order

Prior to this enhancement compiling numpy would forcefully check BLAS/LAPACK
libraries in the following order:

BLAS:
- mkl
- blis
- openblas
- atlas
- accelerate
- NetLIB BLAS

- LAPACK
- mkl
- openblas
- atlas
- accelerate
- NetLIB LAPACK

This is problematic if a user want to build using, say, OpenBLAS but MKL is installed.
Even populating the site.cfg correspondingly one would get a successfull build, but
using MKL, if present.

The same applies to OpenBLAS vs. ATLAS etc.

Especially for developers this may be desirable to check performance with various
BLAS/LAPACK libraries.

This fixes the above issues by enabling users to forcefully set the order of loads
via environment variables:

  $> export NUMPY_BLAS_ORDER=openblas,mkl,atlas
  $> python setup.py config ...

would first try OpenBLAS (if existing), then MKL, and finally ATLAS.
In this case the build would fail if neither of OpenBLAS, MKL or ATLAS is present.
I.e. this can also be easierly used to test whether a linking would work. This
is because specifying a single library forces only one library check and has
no fall-back procedure (as requested by the user!).

The same applies to:

 NUMPY_LAPACK_ORDER=openblas,mkl,atlas

This has meant that the blas_opt_info and lapack_opt_info classes in
system_info.py has *completely* changed.

Effectively there is only ONE change:

A fall-back of LAPACK was previously using get_info('blas') to get
the BLAS library to correctly link LAPACK. However, this may be undesirable
when the user has OpenBLAS/BLIS/ATLAS in a BLAS only installation but wants
to use the NetLIB LAPACK. Hence now lapack_opt_info uses get_info('blas_opt')
which does change the fall-back routine slightly. But perhaps for an easier build?

Signed-off-by: Nick Papior <nickpapior@gmail.com>

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