build: fixup python search and support BLAS_LIBS, LAPACK_LIBS

Created by: drhpc

This is a set of changes we carry at Universität Hamburg to get the LOFAR toolchain built. I hope it can be included upstream, or at least something equivalent.

Important points are:

• Use current (less deprecated) cmake ways to detect Python 3.
• Less redundancy in Python checking in CMakeLists at deeper level (Always try to build PythonDPPP as Python and Boost is required anyway.)
• Add support for BLAS_LIBS and LAPACK_LIBS, as this is customary in HPC installations to choose the (optimized) BLAS implementation in an environment that offers multiple options.

The last point is repeated in other patches we carry for the LOFAR stack. Maybe the common source for the FindCasacore code should be patched instead?

The motivation for us is to get the build going while being able to select python and BLAS/LAPACK implementations using environment modules.

Edit: Remove Boost::pythonXY motivation, as this is already in current DPPP, the change there is only the way of finding Python 3.