Add support for using FlexiBLAS as BLAS/LAPACK backend
As the support for FlexiBLAS has been added to CMake staring 3.19, CMake scripts can be changed to support it. The only downside is the minimum required version for CMake must be changed from 3.18 to 3.19. Considering CC stack has CMake 3.20 that shouldn't be much of a problem.