BUG: get_info('openblas') does not read libraries key

The documented libraries tag in the site.cfg is not read by the configuration,
instead openblas_libs is used, this is inconsistent with atlas configuration.
So libraries first and then try openblas_libs for backward compatibility.

Also ensure check_libs returns None instead of a dict of empty lists if
nothing is found.
DOC: update openblas fork warning with fixed pthread build info
Merge pull request #4191 from juliantaylor/openblas-info

BUG: get_info('openblas') does not read libraries key