python-numpy: add optional blas/lapack support

python-numpy can be linked with libblas and liblapack, so
automatically do it when clapack is enabled.

[Thomas:
 - Fix issues in the original submission and reformat according to
   Yann E. Morin suggestions.
 - Properly format the "libraries" entry in site.cfg as outlined in
   the example site.cfg file: the list of libraries should be
   space-separated.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>