cblas.h installed from libatlas-base-dev is not a C++ header on recent systems

On new Debian (>=10) / ubuntu (>=20.04), the package suggested for cblas.h/cblas.so (libatlas-base-dev) cannot be used because cblas.h does not contain the extern "C" section. Thus, the file cannot be used to build C++ code.

=> Use libopenblas-dev instead.