Merge branch 'fix/PetSc_MKL' into 'master'

Fix/PetSc_mkl

this fixes blas/lapack detection of the PetSc configuration for systems which have no native blas/lapack installed

When native blas/lapack is enabled, we use this for PetSc (just like before, now we just do it explicitly and not by accident) , when MKL is enabled we use MKL and for all other cases, PetSc will download its own blas/lapack and compile it itself. This might be extended to ACML and other blas/lapack providers in the future but i can only test native and MKL on my systems.

See merge request !740